If you are just wanting to do a quick check, the $30 ones are fine. If however, you want a micrometer than can consistently measure into the tenths of thousands, you are better off getting something like the mitutoyo, starret, or brown & sharpe. These higher quality ones can last a lifetime if taken care of. There are parts available for them and can be calibrated by metrology repair shop.
This site has some good information on purchasing high quality precision instruments:
027 : Long Island Indicator Service : Sales, Repairs and Spare Parts
PS: I've been purchasing stuff for my toolbox so that I can do my first flat six rebuild. I have bought a few indicators and mics off e-bay and will send them to get calibrated. I think I'm still a few dollars ahead than buying new, but if you don't want to mess with that, be aware that mics and dial indicators can be purchased at Amazon for a decent price and if you search, you can get even better deals elsewhere.