Hmmm, 80% is not bad, it means 20% payed above minimum (I run freemium MMOs and 95% never pay anything, not that this model is compatible, but still
).
Comparing Xienen's stats and
http://joostdevblog.blogspot.com/2011/10/proun-is-big-success-pay-what-you-want.html it seems that $20 is the maximum you can expect from a player (he got 2 players paying $50 while total earnings were $19k, Xienen got also 2 who paid $50 while the earings seems much smaller, it seems like the amount of $50 is fixed and related to friends than normal fans, anyway, in both cases there is no point expecting anything above $20 in any reasonable quantity).
Total revenue breakthrough (Proun):
$2-2.99 price point - $4k
$5-6 price point - $9k (he said the $6 was from added tax, so we could count it as $5 I guess).
$10 price point - $6k
$20 price point - $1k
Surprisingly there were basicly no $1 price point sales... Seem as if for those who actually decided to pay $2 or $3 was the lowest. I wonder if this proves something...
Overall, I would speculate that $1 minimum is bad for 'pay what you want' model. PayPal has around $0.50 fixed fee, so $2 is around 3 times more than $1 for the developer (not relevant if it's sold via portals, but these don't allow this model so it is again irrelevant). Also it might seem that $2 is still seen as a very low price by the player.
On the other hand "just $1" sounds better.