In the sub $300 range you're going to have to sacrifice performance with a lower end regulator set or buy used. My preference would be to buy used. Keep in mind that no matter what you get you will eventually have to get it serviced, so check out what shops in town sell. Many shops will only service the brands they sell.
If you decide to spend a little closer to $400, I've heard these are good
Regulators although I've not used one myself. The company has a good reputation. You'll have to mail a deep6 off for service or DIY as they don't have a huge network of dealers to service these. They are one of the few brands that actually encourage users to DIY (after you take their class). Service can get expensive, so that is a big cost savings over time.
Also, you're almost definitely going to also need another second stage (octo) and a guage of some kind on that regulator unless you have some other plan. There are lots of used regulators in the classified section of this website:
Classifieds: Regulators I've had good experiences, but you'd be buying from some person, not from scubaboard so it could go any way (kind of like ebay or craigslist). Also, facebook has some very active used gear "groups". Here is one:
SCUBA Diving: Florida Diver's Forum - Florida Diving there are many more groups. Lots of folks take up diving, buy all their gear when they sign up for class, then stop diving. A year later you see their barely used gear up for sale at deep discounts. Just keep in mind that if you buy used you should always pay a shop to have it serviced before you trust it to dive with.
If you have a scubapro dealer in your town, scubapro occasionally allows their dealers to sell "last year's models" at a substantial discount. I believe I got a scubapro mk25/s600 (relatively high end reg) for $350 this way.
Also check websites. I've had good experiences with diverightinscuba.com and scuba.com. I don't think I've used leisurepro.com for anything significant as I've heard mixed reviews - some say they're great.. others say they're not.
One of the "buzzwords" you'll want to look for is "balanced." You want a balanced regulator.