Lisa, neither one of your questions are easy to answer.
The first question you answered already. There are collectors who collect these by state, by city, etc. They can also collect them by stamp - just collect one stamp and find all the precancels. There are numerous possibilities.
For a regular collector a precancel is worth less than the same stamp cancelled. To a precancel collector some of his stamps can be very scarce ( read expensive ).
There is a precancel society here in the States, and they could help you if you are interested. Go to http://precancels.com
. Hope this helped just a little.