Depends on which worms your pet has. There are different ones and each has a different medication. Department stores dewormer may not work,and actually cause worse problems. Seizures is the main side effect that may happen after using the wormer not bought at a vets office. The liquid dewormer at our office is about $6, then it goes up from there. Revolution protects against all worms, but it doesn't cure, its just a preventative.
The answer depends on the parasite. Whether it's round worms, hookworms, ringworm, heart worm or tape worms. Most worms can be killed with Pyrantel dewormer which is about $20.00. Some tough parasites or a combination of these will require something stronger and that cost more. Hope this is some what helpful.
Well.... Where I liv its about $20