Bamrungrad hospital business center does it for 700. Some agents charge up to 1000 but pickup and delivery at your door. I think 500-700 are the standard prices.
FYI, banks will never open any account for you. On the other hand agents could arrange it but you have to enroll in any of their yearly visa package starting from
*****
THB. Agents will not help you with opening bank account alone.
Question to yourself, is she returning? If so, after her next visit which is very likely during the regular 10 year B1-B2 visa, would she be always returning?
You need some preparation by writing down all dates your money wired in. The bank will pull out foriegn transfer records on those days more quickly for you.
if you physically live with your Thai spouse and have registered your marriage in Thailand, I see no difficulties regardless of nationalities. Thai Marriage license set, 2 month deposit of 400K in Thai bank, some family photos, Tm.30 set, Thai spouse ID set. In Bangkok office, make all paperworks 2 sets, one for the extension another set for a random visit officer.