I've been spooking around on tech sheets trying to find out if the Boss Br-8 provides phantom power, and I find no evidence that it does. Correct me if I'm wrong. Unless it does, you need an external phantom power source, a tube mic, which has it's own power source, a preamp, or a good dynamic mic, which doesn't need it.
I'll let the techno-geeks here give you the babble, but basically a condenser, or capacitor mic requires an external power source, and tend to be more sensitive than dynamic mics. If I was you, and I didn't have phantom power, I'd probably just get a good dynamic, say Shure SM-7. And as far as the $300 vs. $500 mic, what can you tell me about the $300 vs. $500 guitar?
You can't be sure, can you? Some people like Fenders, some Gibsons, and some use both, for different songs. And some guitars are just better than others that are supposedly the same. Vocal mics are particularly tough, because certain voices just *go* with certain mics, and not others. Even some very inexpensive mics sound great on certain voices. Later, you'll learn what kind of mics make your voice shine. At first, I think you do well to get a mic that's good enough so that even if it isn't the greatest mic on your voice, it will always be good for recording *something*. I think the SM-7 is one of those, and it's likely to sound good on a wide variety of voices. Although it's the dynamic I'm familiar with, there are models by Sennheiser and EV which are loved by many on this board.-Richie
but not the mic pres. 
