Based on averge retail markups it costs around three times the price of a thing to build said thing from its own spares. For example if you buy a gun that retails at £100 it will most likely cost you around £300 to build the same gun from its own parts bought as spares at retail price. Now, taking into consideration the fact that if you were to build a gun from scratch you'd probably choose higher quality parts then you could easily be looking at 4-5 times the retail price of an off the shelf AEG. Probably more if you use Madbull parts.
Of course there's then the issue of parts compatibility, fit, testing, reliability etc etc etc.