I think average $ per join is a basically meaningless number. What matters is revenue per click --- or even better -- revenue per banner/ad impression ("RPM" ala dollars.com).
Would you rather get 10 joins in 10,000 uniques (1:1000) when the joins are $50 each ($500 from 10k clicks -- 5 cents per click), or 20 joins in 10,000 uniques (1:500) at $30 each ($600 in 10k clicks -- 6 cents per click)?
Avg $ per join looks
a lot lower with the $30 joins, but which is really making you
more money from your traffic?
On the other topic, programs that offer high temporary payout promos are gambling that webmasters will add lots of links to take advantage of the promo, then not remove them all after the promo is over. Its often a loss-leader to get more buried links. Also, gives you a way to get in front of everyone on the boards without a new site or product, because nothing gets webmaster's attention like saying "$100 per join". It's all
marketing games.
There are lots of other intangibles (like support, speed of payment, payment frequency, payment options, marketing tools, etc) that should be considered when deciding on sponsors to promote, but those things are often overlooked by people chasing a big payout.
10 cents worth of

advice
Steve Lightspeed