AR-15 on a budget
Starting out with a kit like this is $289 from www.deltateamtactical.com.
A bolt carrier group which is between $60-$80
The only thing you will need is a stripped lower such 
as this one. You can get them for $34.99 so minus the tools you can build a complete ar15 for $384 minus shipping and FFL transfer fee.
I will be posting step by step and some videos how to assemble a kit like this as soon as they come in.
Do they have everything needed for an ar10?
Yes you can find ar10 kits. They are a little more pricy though.
http://palmettostatearmory.com/psa-pa10-18-308-win-1-10-nitride-mid-length-rifle-kit-516445787.html this one is a AR-10 kit minus lower recover for $399 free shipping