Prey can be purchased if more than three PC's - will be about £1.20 each for your 50 machines
Also, it installs a guest account which the 'thief' will tend to log onto first as it will not be password protected. This guest account will have no admin rights so the program can't be deleted - its not easy to find anyway by the average person
The idea of Prey is really to protect your data, with an added bonus of the potential to get an image of the thief and location details
Yes, the machine could well be just formatted, but in almost all cases a thief will try and browse the machine first. A BIOS password will make things difficult for formatting too
So, when used with encrypted data, then the loss will just be of the machine, but with a potential bonus of tracking and identifying the thief