Well I don't really know 100% how people deal with credits now, but from what I can see it seems like they charge more in credits now since they're not worth as much for us users as they used to be.
As long as you don't use support tickets at least.
But then with that they can't access the money until they have at least $50 in their developer wallet.
So I think if someone chargers like... $10 for a DP they'll normally probably charge anywhere from 16,000-20,000 credits.
Some people might do it the even way, but people that prefer paypal will charge more for credit payment. I've seen some prices be converted completely messed up though honestly.
I guess really it depends on the individual xD But I'd say the ratio I put would be pretty fair for everyone.