Basically, build a NA motor with a stout bottom end that will make around 400 hp. Then, and a 100 shot of nitrous and you'll have the cheapest 500 hp you can get. You can build a strong 400 hp motor for around $3000 using forged pistons, a stock forged crank, and aftermarket forged rods. Summit has really improved their brand of forged rod in the last year, their stage 1 bushed rods should be good for 500 hp on nos and only cost $230 for a set of 8. Once you've got your 400 hp naturally aspirated established, add a 150 shot of nos or something and you'll be at your mark or above.