Building your own it tough. It requires special tooling and lots of time. To keep the cost down you will have to keep your eye out for places having specials on parts, and buy them when they are low. Honestly I think building one is good, but (to me) it would be more for the ability to control the parts being installed more then a cost savings. You can get the complete gun YOU want. Instead of having to buy something "off the shelf". I wouldn't try building your first one. There are more moving parts then you think. It's not as simple as buying a barrel, upper, lower, and a trigger.
With the market being flooded with some really amazing guns. You can buy a quality factory carbine for cheaper then even before. Buy local! Check out the Windham Weapontry AR-15's. Awesome guns. Can be had for around $750 (iirc). Buy online! You also have some great guns available. PSA (Palmetto State Armory) puts out a new M4 with amazing reviews. You can buy it for $699 I think. For $1000, you can buy it with the same Aimpoint scope installed that I have.
I have dreams of building my next one. But now It's because I know the ins and outs of the gun. I will be building a purpose driven gun (probably in 7.62). It wont be to save me money, because it probably wont, but so that I can get what I WANT.