Hi guys. I'm thinking about getting a cheap ($20 or less) mike, or adapting one I already have, for online voice / etc. I'd like a headset type mic, but I already have a fairly decent pair of headphones (Panasonic RP-HT355). (The 'phones have been discontinued, so if I got another pair I'd want one that's at least as good - these were $30 new.)
I used to have an old computer desktop mike, but I don't think I want to use that - too poor quality. I am willing to pay more than the $5 or $10 some units go for (if I have to get a new mike) so I don't get absolutely crappy quality.
I do have a couple Radio Shack dynamic mikes laying around, and my dad has a Sennheiser with an XLR connector (but I'll probably use my own mikes), and I just bought a Zoom H2 (which I'll use for recording purposes when I'm not at the computer, but could I use that too?)

One thing, which I don't know how easy/hard it is to do... get some setup so that I can talk in a normal tone of voice with people in the same room, and they won't hear me. My headphones don't really leak outside sound much if at all, and I'd like the mike to be the same way. I don't need a really good mike... but I don't want something that sounds cheap either. Something with good dynamic range (>96dB), frequency response (27.5-14kHz), (and yes I DO expect to play sounds that low ... I may somtimes play piano on the mic, and I frequently use the lowest notes on the piano (27.5Hz in the case of mine) and I want a well-defined fundamental that's at least as good as the one the piano produces live (it's an upright, but comparable to about a 6'8" to 7'3" grand piano or so)), etc would be nice, but I could go with less.... just not a mike with an absurd 100-8kHz response!

What do I need under $20-25 to get set up? I'd prefer, if possible, to use something I already have. (my bro has a headset+mic across the room that he's not using, but i hate the street-style headband and the ear pieces have no pads, so I can't wear them for more than about 0.2 seconds without intense pain.