I've looked at CyberPower's pricing and it's very reasonable for upgrades to their stock systems. Unlike Dell, etc. they don't charge crazy markups of 2x - 4x over Newegg prices for RAM, hard drive or video.
iBuyPower is about the same, though I think I saw more name-brand parts choices at CyberPower the last time that I looked.
You're probably not going to find cheaper prebuilts where you get to mix and match parts for the best value. HP, Lenovo, Gateway etc. all only offer very restricted combinations, like making you buy an i7 processor (not useful for gaming) before they will let you pick a good graphics card.