It depends on the rank of your account on bitcointalk, obviously higher rank means higher earning due to the payout being higher
Looks like YoBit pays the highest. I'd be happy with 0.03 BTC / month posting here and there to be honest!
I have looked at the rates, and if you post every day, then YoBit pays you
BTC0.18 per month. That is a huge amount, and it is equivalent to $255 at today's exchange rate. There are a few others, which pay around
BTC0.035 per week, which is equivalent to $225 per month.