OK, I reduced the maximum to the same price as copper membership, since it doesn't really make sense for copper membership to be cheaper when it includes whitelisting.

In the last 30 days:
 - 16441 users registered.
 - 4040 required a fee, and the average required fee among them was 0.00127297 BTC.
 - 99 paid the fee, and the average fee paid was 0.00116449 BTC.

I'm not necessarily bothered by the number of accounts abandoned due to fees, since many of those will probably be spammers searching for a no-fee IP. The fees actually paid (by people who are presumably not spammers) may be a bit high, though: I feel like it should be about a tenth of that.