Not having the money, or if you're simply not wanting to part with the money, for a name brand crossbow doesn't mean you can't have a good crossbow. If you've got some basic woodworking skills, make your own. Once you make one, you'll figure out how to make it better then next time. And, if you're truly in a survival situation, you'll know how to make one from materials available when the chips are down and you really need one.
You can buy parts and/or look at basic crossbow plans here:
Instructions on using a binding to fix the prod to the stock here:
You can buy the steel bow from Alchem or make a bow of wood. I've got some elm seasoning for a new bow for one of my crossbows right now, should be ready to go in another couple of weeks. Osage Orange (aka bois d'arc, bowdark, or horseapple tree) would be better, but I just happened across this wood that was going into a bonfire if I hadn't rescued it...
Here are some plans for a more sophisticated crossbow by Chester Stevenson from December, 1953 issue of Mechanix Illustrated
. Ignore that subtitle on the first page, the bit about "Old time weapon has the hitting power and accuracy of a modern rifle." That bit is pure myth, and myth that the crossbow world has been fighting ever since.