These 'timing lights' have a knob on them, calibrated in degrees and they will retard the flash by the number of degrees dialed up.
You can use it to check you auto-advance and vacuum advance in you distributor, by running the engine up to different speeds / loads and bringing the 'flash' back to TDC and reading off the angle.
Also makes say setting your inital timing at say 1500 RPM easier. Say you need 6 degrees at 1500 RPM, then set you timing light to 6 degrees, bring the revs up to 1500 and time the distributor to 'appear' to be at TDC.
Not 100% sure on price range. The current one I have was around $150.00. I would also suggest if you are buying one to make sure you can get spare tubes as these do not last. The first one I bought was a bargin at $45.00 but the tube only lasted about 8 weeks and I couldn't get a replacement.
Regardless of how many $$ you spend,iIt is also worth checking it's accuracy and repeatablility. You can do this you self checking against the timing marks on the flywheel (accuracy) and then adjusting the knob say full range and then bring it back to see if it has the same 'reading' (repeatability).
