The lowest cost legit (501st approvable and not recast) is ATA Hips styrene armor. As stated above, if you can do some research on Fisd, you can track it down.
Armor makers only make armor, there are other things you need to complete the costume such as the following (I have indicated where you can save money by making your own instead of buying)
All told, you "could" build a 100% legit and approvable Stormtrooper for around $600.
Eg:
- HIPS armor unpainted (add the cost of paint for glossy look)
- canvas belt (sew your own)
- neckseal (sew your own)
- black under suit (use cheap thermals instead of brand name)
- holster (sew your own)
- black gloves
- boots (TKboots are about $60)
- strapping nylon, elastic, velcro or snaps
- blaster (diy or Hasbro painted black)
- paints, glue, misc tools
You shouldn't need to buy anything on ebay. All legit sellers of armor and accessories can be found on Fisd. Most importantly, ask your local garrison for help. If you need larger armor, look into RT Mod.