s0ulzer0 is right. You could save money by using an Olympic Arms cast lower, but an RRA is probably a better choice for your first project.
Stick with RRA parts. You might come in a little over budget, but you'll have a better experience.
If you want save money and log some time learning how to troubleshoot the AR-15 rifle. Then I'd suggest that you get an Oly cast receiver (+/-90). You may need to open the magazine well and bolt catch recess. Combine this lower with an M&A Parts post-ban kit (+/-$425). Factor in another $5 for a pivot pin detent installation tool and perhaps another $15 to replace missing or defective pins, detents, spring etc and you're there!
Have fun.
A lot of hobbyists wait until they have a bunch of extra assemblies and parts (shot out barrels, stock internals that have been replaced by 2 stage triggers, old buttstocks, stripped uppers etc.) in their parts bins, and then they build them up as beater Frankenguns on Oly cast lowers. When I lived in the mid-west I used these Frankenguns to harvest my share of yodel dogs on the weekends.
If you want something of better quality, then go with RRA.