Back when it was really cool and unusual to have one, I shelled out $500 for a 12" Mitutoyo digital caliper. One of the absolute best measuring tools I've purchased. (They don't cost that much any more, of course.) In any event, I would suggest that if you go digital, to get an 8" and not a 6"--it will have a lot more usefulness overall, and doen't cost that much more. One of the especially nifty things about digital equipment (at least from Mitutoyo) is that they will work in both Standard and Metric, and will convert values back and forth between systems at the push of a button.
In looking at mechanical calipers, get a pair that measures only 0.100 per revolution, not 0.200 per rev. It's much easier to read, and generally more accruate.
Check the ads in the newspapers. At least once every couple of weeks there will be an ad from Mrs. Machinist selling off the departed Mr. Machinist's workshop. Pawnshops may also be an acceptable source.
When buying used calipers, mechanical or electronic, there are a couple things to remember:
--there should always be a brand name on the instrument..."Central Machinery" does not count.
--the jaws, inside and outside, should grip a piece of paper equally along their length. If they grab more tightly at one point, or you can see daylight between them when the jaws are closed, don't buy them.
--all the edges on the measuring surfaces-- inside, outside, depth-- should have a definate edge. Not so much that you can cut paper or your hands, but not so much that you think "this is a nicely broken edge".
--invest in the $1 or so that a pin gauge of .2500" in diameter costs, or a 1.000" calibrating standard costs. It's worth the small amount of money they cost when buying used equipment.