Try some of the used furniture/junk type stores,flea markets,garage sales, Salvation Army. Repaint restore the frames.
Lindag

I needed a long narrow frame for a project. Found one on clearance with a print in it for $8. Took the frame apart, painted it a color to match my painting, had the correct size mat cut for $9. Put it all back together and ta da! a perfect frame job for $17. I know this doesn't happen all the time when we need to frame a work but when it does it sure is fun.

Have you tried using sectional frames. They come in wood and in metal. You can get the size you need cut to order if they don't already have the size you need. I have used the company below and find that they are very reasonable.
Mima

Here is a link that might be useful: Frame site

I know what you mean about the price of framing. As was mentioned before, sometimes you can find beautiful, used frames and refinish them yourself. I have had some luck with this. Also, you could give the buyer the option of buying the art without the frame. Sometimes people prefer to choose their own frames, anyway.
A lot of people don't realize the the time and costs involved when selling art.