http://www.bing.com/shopping/ar-15-mod4sa-lower-receiver-stripped-mod4sa-ar-15-lower-receiver/p/7EE5D182FE1A5F355003?q=ar+lower&lpq=ar%20lower&FORM=HURE
Ok, you're at $90 for a stripped lower. Keep going until you hit $500.
You need:
A lower receiver parts kit (including trigger, hammer, sear, trigger guard, pistol grip, and all the little pins and springs,) $80
A stripped upper (with forward assist plunger and dust cover. So much for saving money the MP15 Sport way...) $100
A bolt and carrier, $110
A barrel with gas block and sight, $145
I'm already over $500 and I still need a charging handle, gas tube, buffer and buffer tube, buttstock, handguard and delta rings, rear sights, etc. Not to mention an Armor's tool or a castle nut wrench and a barrel wrench.
You get the idea. Even if you were a very shrewd internet shopper, it's still more like at least $650 these days to piece together a carbine from new parts, and even more by the time you add in all of the shipping charges from the various web sites.
I'm sure you have a well stocked parts bin, so a lot of the little stuff you already have on hand. I bet you could pull off a $500 build from all new major parts plus your scrounge bin. But for a first time AR owner, this MP15 Sport isn't a half bad deal.