The price range is a good place to start... now we need to know what kind of rifle your interested in? In-line or traditional sidelock? Perhaps what kind of ignition your interested in also, such as flintlock, cap lock, or in line 209. Do you like a bolt style, striker style, or break open type? What kind of projectiles do you want to shoot? Roundball, conicals, sabots?
Look at a New England Arms (H&R)
Sidekick. They can be had for about $150.00. A scope can be mounted on them and they are easy to clean and maintain. From all the reports I have read on them they are great shooters.
You might also want to look at the CVA and Tradition's line of rifles.
CVA Magbolts are on sale right now. They are not a top of the line rifle but are a good shooter. Also sticking with a .50 caliber is a good idea.
You need to look at a lot of the sites and determine what appeals to you. There is someone here that probably uses that rifle and can give you the low down on one that appeals to you.....