I'm getting ready to do an AR build - here's my take on AR's right now.
1. No matter which way you go - buy one outright or do a build, you are going to have more money into it than it's actually worth. Hey, it's a seller's market out there right now, you're either going to play and pay, or sit out on the side lines.
2. Your best way into an AR is doing your own build. For me, I already have Some AR parts laying around, like a new barrel, sliding stock and tube and a few other misc. parts. Not wanting to part with the barrel, I decided to buy a matched upper and lower. I know I paid too much for the set - $200 over what I think I should have paid. But, someone was willing to pay just $5 less than I did on Gunbroker, so who knows.
3. I want to build a higher end AR, so I chose a quality 7075 Billet upper and lower matched set to use as a base. Many claim this isn't necessary and that mil-spec forged receivers are more than adequate. Personally, I think the heavier billet receivers work better with modern (heavier) rail systems and free-floating barrel systems. If all you are going to have is a set of plastic handguards clipped to your barrel, then the lighter receivers might be just fine.
5. The real "gotcha" is when you start to price out your collapsible stock system, rail system (if you want one of those) bolt, bolt carrier and lower receiver parts. All that stuff adds up, and when you get into the better quality stuff (some claim is not necessary), then your build can get quite expensive.
If you are looking for a basic AR, built on the cheap, they are out there. If you want quality, you will have to do your own build, because you will never find what you are looking for already built in today's market. The manufacturers are selling their receivers, either through affiliates or directly, on gunbroker - they are selling them as fast as they can make them. Gun shops can't get rifles because they aren't buying the components and assembling them.
These are just my recent observations - I'm sure some more knowledgeable people will correct me, where needed.