Im not sure what you mean by mixing uppers and lowers. I think your best deal will be finding one locally thats been hanging around. Many guys grabbing good deals under $1000 in the current economic climate. Armalite and DPMS (includes Bushmaster) will be your best bets. These are not like AR-15's and as a general rule it is not cheaper to DIY a 308AR, it's more about making what you want.