sounds like the bent IOs have pumped the price following the Chinese visa debacle, as expected. so why are you blaming the agency and making false claims about them holding you hostage? you're free to use a different agent any time you like.
18,000 is a reasonable price given the circumstances. in Pattaya it'll cost you 12,500 + 1,600 if your Bangkok Bank branch is not in Patts/Chonburi.
if you don't have a BBL account then it'll cost another 3,700 or 5,000-odd depending on your age to open a new account. I forget the age ranges, maybe up to 60 for the cheaper price (because bank insurance).
90 day reports are 100 baht.
of course you've got to physically present yourself in Pattaya for the renewal and you'll have to mail your passport to the agent for the TM47.
If you're in the south then obviously there are travel costs, so all up you probably won't be better off by using a Pattaya agent rather than your current one.