You get what you pay for, that's for sure. But in this day of digital electronics the digital systems for precise measurements has become so cheap, so easy and so reliable that its pointless to go any other route. It will give you instand inch/metric conversions and is far less prone to stupid operator error.
What you get with the higher quality (and higher priced) units is better wear and tear and smoother operation. If you really need to measure down to 0.001 inches you should get a really good unit.
Knowing HOW to use the caliper and what its limitations are will be far more of a factor than the accuracy of the unit, however. For example, you can't use a caliper to measure the bore of your cylinder, or the end gap of the rings.