If you are looking for a relatively inexpensive upper (minus the bolt carrier group and charging handle) DSArms may have the perfect solution for you. They are selling an M4 style upper with nitride treated barrel for $275. From what I could research on the web, the nitride treated barrel should have a similar live expectancy as a chrome-lined barrel. Just couple this with a BCG from Bravo Company or LMT and a charging handle, and you'll have a complete upper for under $500.

I've heard the turn around times are pretty quick (just a few weeks) but can't confirm it. It's times like these I wish I had more of a budget for guns. 

Here are the specs from the DSArms site:
- Forged 7075T6 A3 Flattop Upper Receiver
- Forged Front Sight Base with Bayonet Lug
- A2 Flash Hider
- 16 Inch 5.56mm NATO Nitride Treated Chrome Moly 1:9 Twist ZM-4 Barrel
- M4 Style Handguards
- Aluminum Delta Ring
- M4 Style Feed Ramps
- Upper Half assembled as shown, no bolt, bolt carrier or charging handle included.

If you want to purchase one, here is the link : http://www.dsarms.com/DSA-ZM4-Forged-7075T6-A3-AR15-Upper-Receiver---DSZM4CBU/productinfo/DSZM4CBU/