Now, this page has some info for making your own lap counting system with some laser pointers, photo receivers, and a pc. Basically it works off the premise of multiple lasers going across the track at different heights. When a car goes thru, you have little flags at different heights on the cars antenna. Each laser trips when the flag goes thru at that designated height, and records the lap times etc...
He does not go into a ton of detail, but it is still a good read nonetheless.
http://www.itsallbutstraw.com/Mini-z...ap_Counter.htm
Then there is this guy who is selling one already built for about $380....
Very high, but I am getting a Mini-Z soon, and I have talked to my local hobby shop, and we may build one for their track....
http://www.mgsweb.com/rc/lapcounter/forsale/Index.html