We recently got into this sport as well, luckily we used our spare coin jar to buy one. Know it doesn't help you right now, but if you start throwing loose change in a jar it adds up quickly. Our local city actually has a loan out program so we were able to get try it before we bought one and were hooked.
Also some places pay you for recycling HP toner cartridges, $2-3 a cartridge.
Check out Letterbox.org to find a intro to get you started until you are able to figure the GPS unit out. Good luck, its a great family activity.
Karen