I don't know that you will find many piston for less than that that will fit your car and not break or wear out fast enough to make you regret it. A friend of mine replaced the pistons in his DOdge Dytona. His dad works for Chrysler and he got them cheap, but he still paid like 85 bucks for them. Unless you have a 12 or 16 cylinder engine saving 10-15 bucks per piston isn't worth it.
1995 E420 - retired
1986 420SEL - retired