There's a pattern here, actually a multi-pattern. I'm generally in agreement that if you get just one mic, and you want to use it for vocals and guitar, your best option is a large diaphragm, multi-pattern condenser mic. The standards of the industry are AKG C414, Shure KSM44, and Audio-Technica AT4050. Less expensive alternatives include CAD M179 and Studio Projects C-3. The most expensive is the AKG, which is also confusing, as it has been around longest, and has the most different versions. It is also my first choice. My recommendation would be the older, but not old enough to be pricey/vintage, C414B-ULS. The KSM44 would be my second choice, with the AT4050 third choice. All are perfectly good mics. The C414 is stellar on acoustic guitar, and a huge variety of other stuff. The new C414B-XLS and C414XLII (same mic tweaked slightly for vocals) is, in some ways the better mic, but is currently out of your budget, basically a $1000 new, and it hasn't been out long enough for used ones to be a lot cheaper. The good news is that the older B-ULS and TLII (same mic tweaked for vocals- you will note that is has a gold grill) are getting a little cheaper on the used market, and are in the upper limit of your price range if you get a little lucky. Assuming they aren't broken, the AKG, the Shure, or the Audio-Technica will get your job done, and you are unlikely to want to re-sell any of them anytime soon. Best of luck-Richie
PS- The Shure SM7b mentioned several times above, is a high-end dynamic mic designed for broadcasting, which is a versatile, and truly excellent mic, but it would not be my first choice for
acoustic guitar or piano. It would probably be the second mic I would buy, after that multi-pattern condenser. Note that any of the mics above will require a preamp, mixer, or audio interface, and the condensers will require one that provides phantom power. If I am talking down to you, I apologize, but I read posts every day from folks asking the question you have asked, that think they can just plug a mic into a computer. You can't.-Richie