I wanted to try building an inexpensive crank forward so I started with an old AMF single speed bike that had little wear on in but needed some TLC to be ready to ride. I straightened the fork a little and trued up the wheels and adjusted everything to get the bike rideable and then tore it down and cut up the frame and added 4" of length to the chain stays and put in a fatter seat tube and seat post at a more laid back angle and added a piece of tubing across the top of the frame to stiffen it. I built a larger saddle and put on some pull back handlebars to give the crank forward type comfort. I reduced the gearing to about 50" by replacing the front sprocket with a 36 tooth sprocket. I'm too tall to fit on this bike properly but it is much more pleasant to ride than the original was.
Cost of this project was probably under $40 counting paint and brazing supplies.