If my cat may have worms, what is the best, cost effective remedy I can buy and administer to him?

You shouldn't be spending a ton at the vet. Reasonably speaking a visit to the vet for this shouldn't run you more than a hundred bucks, and that'd include updating his shots. Otherwise you wouldn't know what sort of worm he has. You can get medicines at any pet store. They shouldn't harm your cat any.

It isn't going to cost a ton to go ask your vet for the meds. If he was in the woods, he probably has/had fleas, which means he probably has tapeworms. Go ask your vet for Drontal Plus, its a broad spectrum wormer that will kill just about any intestinal parasites the cat has. I would however, be concerned that the cat might have giardia, which requires a different wormer from my understanding. This parasite is often picked up when an animal drinks water from a puddle or some other standing water source. Another concern is FeLV (feline leukemia), like another person said, which is transmitted through bites. If you don't want to spend the money to properly diagnose and treat your animal, find it another home or surrender it to a shelter. Not getting an animal medical treatment when it is needed is a crime, its called animal abuse.

Best to get him in to the vet - he needs an exam to see what else he may have come into contact with. That's a long time to be out - he may have fleas and ear mites in addition to worms. Best to get him tested for FeLV as well. Instead of wasting money on OTC wormers that don't work, aren't appropriate and may be toxic, put the money towards the vet visit.