There are many speculators out there buying up as many receivers and rifles as they can afford hoping a ban happens. Then when the rush starts, they can dump them for multiple times what they paid.
That's a big "if". Most assume any ban would include a grandfathering of existing Assault Weapons (not just AR15s). That means there will be a mad rush to own one before the law kicks in.
If you're already in the market for an AR, it's going to be hit or miss to find a good deal, but they are out there. Take your time, do lots of searching online, and grab a good deal when you find it.
If you're speculating there's going to be a ban, maybe 80% lowers would be the way to go? No need to go through an FFL (saves lots of money when buying many items), and they can be stored without HPD registration. If the ban happens, you could sell a few at a profit to fund your builds. I've seen some lowers going for ~$50 each. If the ban fails to happen, you would still be able to get what you paid selling them, if you need the cash.
Buying when there is a threat of a ban will cost you more than when things are quiet. Prices started dropping when Hillary lost. Now they are going up. Wait, or buy? It all depends.