a dyno will tell you how much power/torque, timing, air/fuel ratio and other important things about your engine.also when you tune you can still squeese power out of the motor with the parts you have.basically you run the car on top of rollers and its hooked up to a computer thats telling you what going on,to make adjustments on the engine.the cost depends where you go or find a shop that has a dyno.

Local shops here charge 50$ an hour for just dyno'ing, and 120$ an hour for dyno + tuning.

in california it's about 300-600 dollars an hour depending on what pile of parts u give them to try!