If that is just for the visa and no classes then it is peng maak maak. You tell Fairfax that their visa is expensive since the DTV. Some places offer a 5 year DTV for a little bit over their price. Tell them they know their visa prices is no longer with the times and stop being so stubborn. But knowing how Thai companies work, they would rather go out of business then to lower their prices.
Bro, there is a simple solution to your dilemma. Marry her and have her under YOUR DTV visa. Since she is your spouse, any work done under your roof is legal. Also, any compensation from you is also legal as spousal support.
The only advice I can give you is make sure to rent from a Thai owner. They bought the condo for 30% cheaper than a foreigner and will negotiate more on the prices for you. Stay away from the Chinese owners, they are the worst.