I remember I built a kit .45 that came from Fed Ord for $200 many years ago. The frame was 100% complete (well really 97%). It was alot of work and fitting, and it turned out great, though it was alot of work. It was the cheapest of all parts imaginable. It was alot of work with a frame that was already completed. I would not recommend an 80% anything if you have to ask questions about it. Not a flame, just that the 100% frame was more than enough work to get right. An 80% frame may not be a good idea for a first time build.
You can buy a complete parts kit for $250 from Sprotsmans Guide. Then just go down to the gunshow and buy a frame. Put the thing together. I would think you could find a cheap frame for under $100. You'll have to finnish it somehow as well, so add some money there for even a do-it-yourself finnish. I would think you could have the complete gun done for under $350 easy.
You can buy complete .45 autos for that kinda money, but the experience of building it is worth it. I would think an 80% frame would be even cheaper, but would be alot more work and teh chances of screwing it up are dramacitaclly higher.
Ross