It all depends on what you want it to do, try magfed on the cheap or get a good magfed marker that can shoot first strikes?
Imo the best for the price... almost anything used
But seriously, a used package can save you sometimes hundreds versus new, especially if you buy an older gen item.
If you desire new and modular on the cheap I'd go with a tacamo mkv or mk5 setup with dmags. Yea the milsig mk3 is great, if you have 500 to spend, but a basic mkv can be had for around 200 and 10 dmags and magwell for 80. This leaves a lot of room for customization and vest.