I buy all of my frames from one of the advertisers of this forum, frame destinations. Its metal frame in 16x20 costs little less than 30 dollars. 20x24 will run about 35 dollars. These are top notch quality stuff, not the cheap frame from discount art stores. I use Nielsen profile 15. I cut my own mat to save money. There are less expensive moldings also. Using profile 11 will save you few dollars but I don't like this one as it has less "projection".
While it won't be dirt cheap, I think it'll be worth your investigation. I don't think you want to use cheap frames especially if this is going to be an important exhibition.
I have absolutely NO financial relationship with this company. Just a satisfied customer.