I've seen bolt-on front sights for as little as $35.
Cheapest rear sight you can get would be that Chinese copy carry handle. They're advertised at $40-50 but I bought one new at a gun show for just $25. They're cheap but good for a .22.
I paid $400 for my DPMS but you don't always or often find them advertised for that now. Cheaper Than Dirt is your best place to look.
Good luck finding something suitable in your budget. A .22 AR is a real joy to own and shoot and you'll have a lot of fun with it! I don't regret a penny I've spent on mine. Counting a dedicated lower, carry handle, a scope, a red dot, and a couple extra mags I'm into mine for between six and seven pigs and it's worth every dime. MUCH more fun than any race-tuned 10/22 and almost as accurate, too!