Thats just the initial expense, then you have to factor in upgrading it externally. Of course most people have to put real steel stuff on it, shorten the pcb board, mags, batteries, etc.
The main attribute of a PTW is the detail of realism. Trigger responce, 3 burst compatibility, size in every dimention(aegs are thicker than real M4s,), weight is the same, mags are the same size as real M4s, Also the capibility to put a red cylinder shooting 550fps, and in 15 seconds, you can change to a black cylinder for 330fps.
PTWs have only one limit to what you can upgrade it externally, that is the bank account.