Have you looked online? My husband bought a really nice mountain bike for a steal online.

You mentioned building a bike as well, which could be the cheapest way to do it. My Cannondale SuperSix frame was new, but a year old model and had a blemish on it. My husband got it for a super great price and I don't even know where the blemish is and told him not to show me. (He and his friend had to search a long time to find it themselves.)

That same friend built the mountain bike of his dreams by being patient, watching ebay for all the components he wanted and didn't settle for anything out of his price range. It took a little time, but he ended up with an amazing bike for a fraction of the price it would have been in a bike shop.

You might start with buying just a frame and putting your current components on it and upgrading your components a little at a time. Or whatever bike you get in your budget, you can then slowly upgrade further. My bike has been an evolution of upgrades. I think the brakes are the only thing I originally had and my husband has had SRAM Red brakes in the garage to put on it for a year and a half now!