I think you could easily put together a rifle for $700 or less right now
If you are talking about an AR15 for less than $700 right now I highly doubt that.
Your main problem would be finding a good BCG at a decent price. And what is the cheapest lower (stripped or complete) that you can find that is not polymer?
Sure you can take your chance with a Rguns BCG for $115, but I wouldn't. Stripped Lowers are still around $200+/- with the cheapest around $120-150 for a PSA... thats if you can get them in stock.
My suggestion is looking at a M&P15 Sport for $839. For a starter AR that is hard to beat. Some DPMS AR's have also shown up in my LGS for less than $800 but that is low-end IMO.