After a bit of a search I came to the conclusion that there are actually a limited amount of tuners that work well with mac or have drivers. This has been a bit of a problem in the past. However, you don't need to shell out 300! That's just rediculous.
I just got a pinnacle HD stick for mac (try google) for about 120. Works like a charm. Very easy, comes with software and a remote and is USB and simple and even comes with an antena for getting analog signals as well as digital through the cable.
It has been working great for me.
There are about 2 or 3 other options. All around 70-150 I believe. General with software included they come out all about the same at about 130ish though.
Just google mac tv tuners and you will find a few out there, but they have to be specific to work for mac and most generic ones don't it seems unless you run it with the parallels or bootcamp or whatnot.
Like others have said it depends if the company has an OS X driver. I have sucessfully used a Hauppage WinTV USB PVR sucessfully with OS X and it is recognized as the Mac version of the same hardware. In other words they are exactly the same but with a different name.