Just for land nav.
$20 for a surplus compass sounds like a hell of a deal!!
I have a very old military compass that I wouldn't trade for anything. Even has a radiation warning on it. I'd say get a mil surplus one, if you can find one. They're usually tough as all get out and do everything and then some. I've used mine for impromptu surveying.
The reason I like the clear-bottomed (orienteering-type) compasses over the lensatic compasses is that you need not orient the map to N when determining your course, you just have to line the north needle up with the north-south heading lines. They're also much less expensive. The one I have in the vest I'm wearing right now, I got in 1974 and have carried ever since.