Did the vet tell you if the treatment did one type of worm or both?
You need to be careful with some vets as they will just treat for no reason and take money. Many are respectable vets, but there is always the small few.
If it was for both you shouldnt need to go again for a good few months. And a simple worming should NEVER cost anywhere near $60
Even though only one treatment is needed, the reason for recommending the second injection is because most pets who have tapeworms also have a flea problem. The cat can easily reinfect itself if fleas are allowed survive in the environment via the cat or any other furry critters in the house. The infected kitty will most likely have another tapeworm before the flea problem can be taken care of completely. Flea eggs hatch in about 30 days causing another active batch of fleas and starting the cycle over.
Tapeworms result from cats eating fleas, and it only takes one flea to cause all these problems. The flea could have come in on the cat, another pet, or even your clothes. It takes 3 weeks for a tapeworm to fully develop and start releasing it's segments. Each segment is actually an egg sack that breaks open once outside the body and is used as food by the fleas.
If you have been using a flea preventative on all pets for at least a couple of months, then only one injection will probably be enough to take care of the problem. It was probably just a lone flea and you can treat tapeworms when you see them, because the flea preventative will be helping to keep your kitty protected. If you have not been using anything for the fleas, it usually takes about 3 months of so (longer for severe infestations) of solid protection to effectively eliminate the flea population. There are also foggers and area sprays if they are needed for the fleas, but I've always just used the veterinary flea topicals with great success.
You know more about your situation and what you have been doing, so you should check with your veterinarian and use your best judgment. I hope I've helped.