From what I can gather, most of the dudes in here who use or recommend agents only know of reputable ones, not dodgy ones. So we can only tell you the price for the good agents, should we deign to. Not really inclined to do so as it's been discussed in here ad nauseam.
There was a thread the other day from a BM in Hua Hin who got mixed up with a dodgy agent that messed him around. Perhaps you can find that post and ask the OP how much that dodgy agent cost.