You mention you only found the tape worm segments on one of your cats . . . in the past, when one of our cats got a tape worm, we only treated the one who showed the symptoms, not all of our cats. The ones who didn't show symptoms most likely didn't have the parasite. They don't pass the tapeworm from cat to cat - it needs the intermediary host - in this case, fleas. If the fleas are gone from your environment, why not just give the prescribed treatment to those who show symptoms? It may be that they were the only ones infected. Then, if others begin to show symptoms later, then you can treat them at that time. This way, you are only treating those known to be affected, and if additional treatments are required later, you are spreading the cost out over a longer period of time so it may make it easier to afford.
Conversely, you can ask your vet about the other OTC medication that is listed and get his advice, explaining your financial situation. He may be able to advise you if it is as effective, or if it is more dangerous, or if you do use it what you can expect. It may not be as effective, or it may be the tablets he uses expulses the tapeworm so that you can see the thing is removed as opposed to being dissolved and digested. The thing to keep in mind about all worming medications is that they are a type of poison. Ideally, they are only poisonous to the parasite, but they may also have repercussions on certain animals with lowered immune systems or other conditions, so it is always good to have your vet involved in any de-worming situation.
In case anyone has a question, tape worm medication is specific for tape worms and not roundworms, pinworms or other parasites. If fecal tests show your cats have other parasites, then other medication specific for that parasite should be given separately:-).
Good luck with finding resolution to your situation.