I don't actually think it's a good idea at this point either. Just throwing it out there. A few years ago it might have helped to throttle the influx of new users to a level that would have been more easily absorbed.
Less than that: whatever the minimum amount is that payment providers will accept. It's easy to forget that there are parts of the world where even $5 is too much.
Besides, most trolls aren't going to be deterred by any particular amount of money: the simple act of paying is the real deterrent. You'll repel more trolls with a $100 "cover charge" than a $1 charge, but not very many more: not enough to really be worth the trouble.
It's easy to forget that there are parts of the world where even $5 is too much.
Where in the world, specifically, are (a) people that we, the other members, think would be valuable additions to this community, yet (b) $5 is "too much"? Sorry - I don't know all the economies of the world but your statement just seems a bit hyperbolic.