Usually with the similar charts I've seen for member earnings, they're educated guesses based on how much tax they paid.
That's probably why there's 2 columns there. First column is tax paid, second column is estimated gross earnings.
Also, they usually list figures in multiples of 10,000 yen, so Nacchi's 6,432 there is actually 64,320,000 yen.
Using a rough 2002 average figure of 125 yen to 1 USD conversion rate, that works out to $514,560
For comparison's sake, Yossy's 38,240,000 yen in 2002 is $305,920.
Just as an aside, I'm using USD and US salaries as points of comparison, because they're easier for me to think about.
The figure for current Morning Musume has Ai-chan and Gaki-san at the high end with 35,000,000 yen, and 8th gen at the bottom with their 18,000,000 yen.
In the last few years, the conversion rate has fluctuated around 83-90 yen per USD.
Using a rough value of 87 yen per USD, this works out to $206,896 on the low end up to $402,298.
While these numbers might seem a bit low in comparison to famous western music acts, you have to keep in mind that the Japanese music market doesn't have the same global reach.
Additionally, most of these girls are in groups so any profits that their act gets are split between them.
Hamasaki Ayumi, being a solo artist, obviously would get paid a bigger share of her profits.
Without comparing to western artists or top solo acts, I think $200k up to $400k a year is pretty damn good for girls in their teens and early 20s.
Sure the figures have dropped since the boom times of 2002, but that's to be expected, given the increased competition.
But I think it's good to keep it in perspective that these girls are getting paid what would be in the US, 6-figure salaries.
They work hard for the money, and I think it seems like a fair amount.
As long as they're still making good money, I don't see H!P dying out anytime soon.