Expect to pay $20 for some Chinese calipers or $100 for American.
The quality of the dial calipers is not as important as the skill in using them.
With practice, you may be able to measure any pin gauge and determine it's size to the nearest .001"
If you get really tricky with a dial caliper, you may be able to tell if it is a "+" or "-" gauge, that is +.0001" or a - .0001". That trick requires everything to be done right:
1) Calibrate [screw on the dial face ] the calipers with a pin gauge very near the size to be measured.
2) Wipe off the jaws
3) Get the jaws square on the gauge to be tested.
4) Get the test object at the same location on the jaws.
5) Use the same force as during the calibration.
6) Find the spot and the force of greatest repeatability
7) Not every $20 Chinese dial caliper is capable of this, but some are.
8) Don't drop the calipers.
9) Get good by using your dial calipers for competing with machinists with their micrometers.
Get two sets of pin gauges, .0600" to .2500" and .2510" to .5000"
http://www.use-enco.com/CGI/INPDFF?PMPAGE=232&PARTPG=INLMK3&PMITEM=240-0142
Practice measuring the reference pin gauges with the DIAL calipers until your eye can interpolate to get .0002" accuracy on outside measurements.
Dial calipers are never very accurate on inside measurements. Use the pin gauges for that.
With the pin gauges you can measure:
1) the donut inside the case neck at the base of the neck.
2) the neck diameter of sizing dies
3) the neck diameter of chambers
4) the diameter of primer pockets
5) the bore diameter of barrels