Fleas

Does anyone know of a cheap flea treatment for cats that actually works? My cat has been an inside only cat for the past 5 years and somehow got loose while I was bringing the groceries in. He finally came back but is covered in fleas after only a week. I don’t have the money at the moment for a 50$ flea collar, so please tell me someone has any suggestions? I would really hate to bring fleas in my house and get an infestation. 🙏🏼