My first AR started as cast Oly plinker lower for $87 and a Tapco M16 kit for $287 which included shipping. I ground down the M16 parts/bayo lug to make them into ar15 parts for a grand total of $374. It is a darn good shooter.
Another one was a fully assembled Cav Arms lower that I got through here on sale for $175. I found a used post ban 20" HBAR Bushmaster upper at a gunshow for $250 which included the bolt and handle. Total $425
I also built a rifle using a Continental Machine (generic) lower I bought for $100, plus a lower kit $50, plus a used stock/buffer $30, Colt M16 bolt/carrier ground down to AR15 specs used for $50, and a Colt SP1 large hole upper from ebay for $160 made into a post ban. Total was $390.
All are very functional and fun to shoot, if you look around you can buy used parts pretty cheaply.
Oh, in case you think I just have cheap beaters, I have also put together some pretty expensive guns, but don't tell my wife.