So, which should I get and how much does it cost?
Or should I just get on of those you can download and put on your mp3's?

I've got a $20 Sabine Metrotune that goes in 4bpm increments. It's decent but I'd rather have a nicer one that goes up in 1bpm increments and has a louder headphone out.

They all have quiet headphone outs. Get a headphone amp like a presonus SP4 or just run the metronome through your Firestudio as a headphone pre. Or both for super-pre.

That would deafen even the most foolhardy drummers

If you've got a computer/studio set up, then I'd just use the metronome built into the DAW. That's what I do for practice.