Things you'll want to look for in UTAU mics are:
- large-diaphragm condenser microphone (this is the best mic type for clear vocal recordings, essentially)
- a cardoid pattern (as opposed to omni, etc)
- USB format, unless you are willing to spend the extra on the preamp required to utilize XLR mics
- Depending on your gender/voice type and singing note, the human singing voice varies between 20 and 1,000 hz. You'll want to look at the frequency response chart for any microphone you're looking at, and make sure the microphone doesn't cut out any frequencies essential to making your voice sound 'clear' (for example the Blue Yeti sharply rolls off starting around 100 hz and makes a lot of lower and speaking tones sound 'muddy'. Personal experience with the mic definitely makes my altos and below sound terrible.)
Those are some tips I can think of just now. Some fan favourites for $100 and below include:
- CAD U37
- Samson CO1, Samson Meteor
- AT2020u
- Blue products (Snowball, Yeti, Nessie)
- MXL USB006
- Behringer C1u
I'd highly recommend listening to demos/comparisons of mics to make your decision. At this price range it really starts to come down to personal preference. Best of luck in your microphone hunt!