Whats the cheapest I could buy or build a AR15?

Oly Arms Plinker from FAC for like $495 plus S&H and transfer.

Or a RRA receiver off of the EE for $85 plus S&H and transfer and a $400 kit from Model 1 Sales.

$89 stripped Stinger Arms lower from the equipment exchange.

$20-50 transfer fee to your FFL.

$400 kit from Model 1 Sales.  

Too slow Eswanson, toooooo sloooooooow!

Yeah, but at least I didn't suggest something with a cast lower!

Plus, I get bonus points for being thorough and including the transfer fee.  

Isn't that an oxymoron?

E-95

Well, I priced a quality AR at $625, and with all top quality components.  
Complete RRA lower from local vendor: $200
Assembled RRA A3 upper: $90
RRA Bolt & Carrier: $110
RRA Charging Handle $15
Bushmaster Post-ban M4 barrel, complete: $210

Total: $625

Course, this kinda requires that you have knowledge of the AR system, and assembly, as well as the tools needed.  

Ghost

Yes, but I gave multiple options... score bonus point for me.

I offered an option that has a warranty, even if there is a cast receiver involved... score additional bonus point for me.

You assumed he will have to pay a transfer fee and guessed at the amount... I stated there was a transfer fee, but maybe its zero or maybe less than or more than the fee range you stated... we'll call that a wash.

I win!

Another option is to order a complete RRA lower complete for $190 plus S&H and any transfer from one of our vendors in the EE.  (I'll have to see which Vendor had them listed for that)  And then just order a complete RRA upper from Adco for around $350 and then you could have a complete RRA rifle...  Now that's cool.

I recently put together a Rock River lower, and Model1 kit combo. While its not as nice looking (front sight looks alittle cheap) as some other guns it fit my budget and fires well. $400 for the Model1 kit plus $18 S&H, and $85 for stripped RRA lower plus $15 FFL fee. A friend of mine picked me up a Okay Ind. 30 round magazine from a gun show for $15. Now I can spend the rest of my money on ammo and lots of it.