If you cannot afford a safe, check out this eBay store that has locking gun cabinets for reasonable prices:
http://stores.ebay.com/The-Safe-Guy-Online
A trigger lock is easily defeated, and the gun is still portable with the lock installed so is easy to steal. Burglars know all the hiding places for guns and will look there and find your gun. These cabinets will slow them down but are not as good as a safe. Properly installed a locking gun cabinet cannot be easily removed so burglars will not be able to take your gun.
Just be forewarned that these cabinets can be defeated with pry bars in a couple of minutes, so install them where a burglar cannot get leverage to pry them open!