From Exciting MAX! magazine.
While the actual contract payment amounts are never truly released, I remember years ago a similar estimation coming out during MM's Golden Age, and their guesses were based off of tax records.
I've converted all of the money to USD below, and for the 48G members, I've also included their ranking in the recent Sousenkyo.
50. Erika Denya, Babyraids, $88,830
48. Suzuran Yamauchi, SKE48 (69), $98,700
48. Sakura Miyawaki, HKT48 (11), $98,700
44. Aya Shibata, SKE48 (15), $108,570
44. Ami Maeda, AKB48 (70), $108,570
44. Miyu Yoshimoto, X21, $108,570
40. Miyuki Watanabe, NMB48 (18), $118,440
40. Ayaka Wada, S/mileage, $118,440
40. Kaori Matsumura, SKE48 (17), $118,440
40. Nana Yamada, NMB48 (29), $118,440
35. Rie Kitahara, AKB48 (19), $148,050
35. Pour Lui, BiS, $148,050
35. Akari Suda, SKE48 (10), $148,050
35. Mogami Moga, Dempagumi.inc, $148,050
35. Hitomi Miyake, Idoling!!!, $148,050
33. Sae Miyazawa, SNH48 (12), $157,920
33. Momoka Ito, Fairies, $157,920
29. Makoto Okunaka, PASSPO, $167,790
29. Mizuki Fukumura, Morning Musume, $167,790
29. Maimi Yajima, C-ute, $167,790
29. Yurina Kumai, Berryz Koubou, $167,790
26. Ai Okawa, Idoling!!!, $177,660
26. Hinako Kinoshita, himecarat, $177,660
26. Yui Yokoyama, AKB48 (13), $177,660
25. Rina Ikoma, Nogizaka46/AKB48 (14), $197,400
22. Momoko Tsugunaga, Berryz Koubou, $236,880
22. Riho Sayashi, Morning Musume, $236,880
22. Rurika Yokoyama, Idoling!!!, $236,880
20. Minami Minegishi, AKB48 (22), $256,620
20. Jurina Matsui, AKB48/SKE48 (4), $256,620
19. Reni Takagi, Momoiro Clover Z, $286,230
17. Ami, E-Girls, $296,100
17. Momoka Ariyasu, Momoiro Clover Z, $296,100
14. Haruka Kudo, Morning Musume, $325,710
14. Umika Kawashima, 9nine, $325,710
14. Ayaka Sasaki, Momoiro Clover Z, $325,710
13. Sayaka Yamamoto, NMB48 (6), $345,450
12. Haruka Shimazaki, AKB48 (7), $414,540
11. Rena Matsui, SKE48/Nogizaka46 (5), $424,410
10. Yuki Kashiwagi, AKB48 (3), $444,150
7. Shiori Tamai, Momoiro Clover Z, $454,020
7. Sayumi Michishige, Morning Musume, $454,020
7. Mayu Watanabe, AKB48 (1), $454,020
6. Ami Kikuchi, Idoling!!!, $483,630
5. Kanna Hashimoto, Rev. from DVL, $493,500
3. Kanako Momota, Momoiro Clover Z, $523,110
3. Minami Takahashi, AKB48 (9), $523,110
2. Haruna Kojima, AKB48 (8), $582,330
1. Rino Sashihara, HKT48 (2), $671,160
Well, even the lowest amount here is more than I make, so that's something.
I've heard that female idol contracts in Japan go something like this: (no source, sorry) idols receive a low salary from the office. Some of the idols that don't work out just end up doing office or staff work, if they can't schedule any activities.
However, this also means that they do not earn royalties or anything off of music-based promotional activities: recording, rehearsing, shooting MV, music sale royalties, etc. They may or may not receive a tiny cut from promotional gigs, performances on music shows and such. It's unlikely.
So, basically, income is all about the solo activities. (endorsements, modelling, fashion products, radio and TV gigs as regulars) All you have to do is look at this list, which includes a few podunk groups with single members doing a lot of solo stuff. (X21? himecarat??) See also how some "more popular" groups that don't do much solo stuff make no appearance on this list, like Babymetal or "good enough for Budokan" Tokyo Girls Style.
Interestingly, the non-Sayumi members of MM have complained that Sayu's the only one that gets TV work, so UFA must pay them enough well enough for group gigs (concerts, CMs) and photobooks, and I've always heard that H!P's true bread and butter is concert merchandise. Same with the other H!P members on this list, considering that Momoko is the only one with a current variety career, so the other three (Yurina, Maimi, and Wada Ayaka) must be banking on their modelling.
Consider the 48G rankings. Reigning champ Mayuyu is earning far below Sasshi, who has much more TV presence, as well as management duties that would earn her royalties. Maeda Ami's right there next to Shibata Aya, and the latter does barely any solo activities. (that's an SKE thing in general) Some of these do make me question the numbers, though, as it's weird that Milky is so low, and so much lower than her fellow NMB ace Sayanee.
More importantly, BBQ aka Matsumura Kaori commented on this article, stating that she definitely doesn't see this amount she's supposedly earning. She says that she sees about 5 million yen more-or-less, ($49,350 USD, which is less than I earn) and definitely less than half of the 12 million she's listed for. Fellow SKE member Takagi Yumana teases her in the comments, asking her to buy a house for her, and BBQ grumbles that she can't even afford a bicycle basket. Google Translate also does her next comment as "business interference Seriously."
Former MM 1st gen stars Natsumi Abe and Iida Kaori have also said that during the MM Golden Age, when they were the top stars in Japan, they were still living like beggars. I always thought that it was due to poor parental management, (as they were still minors at the time) not sending them their paychecks properly, given some of the information below.
Some other resources for context:
AKB top member earnings in 2013
AKB top member earnings in 2011
Discussion on H!P payments for non-popular members and graduates in 2006. Mentions monthly salaries, paycheck raises based on seniority/tenure, and the company paying for some of the meals during work. (I have read some stories about younger MM members bringing their own food as well, though. In the other direction, early in AKB's career they pointed out one member who took company lunch leftovers home for the family. See also this, where they're mortified at the Kamei family's penny-pinching. This was also possibly at the height of Kamei's popularity.) This post has monthly salary numbers from what may be their contracts. Note that said amounts do not include the money they get from gigs, but technically, even if they book no gigs or sell nothing, they'd get this amount from the office.
In 2008, Ueto Aya was the top CM Queen with $3.6 million USD.
Putting that $3.6 million USD in context:
Fees per CM in 2012, showing that idols are still relatively small-fry on earning power
CM exposure ranking, by screentime, 2012
Alternate 2012 CM ranking, by number of contracts 2 AKBs on top.
2013 ranking by number of contracts. Paruru must not have as big of a per-CM fee, and she's still mostly doing AKB-associated activities even solo, whereas the others have gigs to their name only.