I'm looking to build a 18" rifle. What's the cheapest route to go an upper minus bcg and hand guard

Build your upper around this barrel:

http://www.bravocompanyusa.com/BCM-18-SPR-SS410-Barrel-Rifle-Length-Gas-Ionbond-p/bcm%20ss410%2018-8%20ib%20bk.htm

Depends? Do you have the tools\knowledge to build the upper yourself? Do you have any specific parts you want used (i.e. barrel, upper receiver, rail or other FF handguard)?

Obviously if you have the tools\knowledge to do it yourself then that will probably be the cheapest route but not always. If you are going to have it built for you then it somewhat depends on what components you want used but in general, ADCO would be a good shop to talk to about a custom, no frills or BS precision build

Well typically SPRs use railed handguards or PRI tubes and most of those options have their own barrel nut which would mean "an upper minus bcg and handguard" is just a receiver and barrel, though obviously those are parts that require assembly. Sorry if I'm misunderstanding the red part.

If you're comfortable building your own, I'd go that route, it's very easy. Otherwise take a look at some of the web sponsors' offerings like Bravo, ADCO, MSTN(not cheap), etc.

I had wanted to find a barreled reciever. Originally, I had planned on going with bcm, but the only 18" I saw there was the entire mk 13 upper.

If I can't get that, i'd guess I'd be stuck with building, which I don't think I'd have a problem with. I've built 3 m4's, I've just never messed with anything free float.

I'm still not understanding what you mean by a barreled receiver To attach a barrel to a receiver you need a barrel nut but most FF handguards have their own barrel nut so you'd need to have the handguards already. If you had a barrel installed on a receiver with a std barrel nut, you'd have to remove it to install your FF handguards. Do you know what I'm saying? (I'm not trying to talk down to you at all so please don't take it that way. Your post just sounds like you're missing something.... or maybe I am.)

Here is an entire page of BCM 18" barreled uppers, most of which you can add options like charging handle, BCG, and sights. It doesn't get much easier than this.
http://www.bravocompanyusa.com/SPR-Mk-12-Upper-Receiver-Half-Groups-s/131.htm