Well check out Open Access. Its a arduino based board to control a security system (RFID doors, etc) that is open source based. The “board” is $200, which if you look at solutions from other companies, is pretty cheap. The board itself looks pretty simple, so you could send the layout to a shop and build it yourself cheaper. You can incorporate it with an Raspberry Pi to extent its functionality as well (ass seen in the picture). What I like about this, is its “open” so if you want to upgrade it, modify it, etc etc. You can. Instead of paying the vendor/distributor for you existing security system an arm and a leg to do any upgrades (think thousands just for hardware)
UPDATE: Got this from a reader (ebay link below). Apparently you can buy stuff pretty cheap that does the same thing from China direct. Thought customization and longterm support maybe very limited. But for 1/2 the cost…..
project http://code.google.com/p/open-access-control/