Frontline or Advantage is cheaper than trying to get rid of fleas that are in your homes carpets. The cheapest way to buy any of these treatments, is too buy the biggest dose you can. As is buy the dose recommended for the biggest dog. I also know that Frontline is the same product for both cats and dogs. Then pick syringes, and a couple of needles at the feed store, and create the right dosages for each pet. Treat all the pets indoor and outdoor if the pets mingle at all. Otherwise you are just reinvesting the treated animal.
The cheapest thing you can do it to keep the animals out of your house, and keep them as healthy as you possibly can. If you can't afford to take care of the pets you own, then you need to make some hard choices. Part of caring for a pet is keeping parasites off of them, and keeping up their vaccinations and heart-worm medications. When you can't afford to do this, then you may need to either find more income, or own fewer pets.
Getting fleas out of the house even when all the animals have been removed can be difficult without expensive chemicals. You will need to wash all the bedding that any animal has been on. (I would do so every 2 to 3 days for the first week, and at least twice week for another month.) Then you will need to vacuum all the carpets, with a real working vacuum. (Many people do not own a vacuum that will doing anything but make noise, let alone one that will suck up a flea.) But vacuum, vacuum, vacuum. Did I mention vacuuming daily? Vacuum the pets if they will let you. Don't forget to empty the vacuum, daily if you can. Put it outside if you can't. You now get to do this for at least the next month.
The reproductive cycle of the flea is about a month. You should notice a difference right away, if the vacuum is any good. You need to clean all the way to the edges too. So pick up anything laying around, every soft surface needs to be vacuumed. This means chairs, sofas, beds and toys. Fleas will hang around as long as there is a food source. You are a food source. Getting rid of all the pets, can often make a flea infestation worse.
I know that I have contradicted myself in saying that one should think about the number of pets in a home, and now I am saying don't get rid of all the pets in the home. It's is about finding a balance to what you can afford, and what your heart wants. (of course at the moment it's currently about what the fleas want vs. what humans prefer.) lol
I hope that this helps some, as for the correct dosages of Frontline, check online sources. I have often found sellers of Frontline products on the E*ay web site often mention the correct dosage per pound of the animal. For some reason I seem to have to look this info up every time.
Even if you buy the stuff at your local vets office, it's much cheaper to buy the big dog dosage package make smaller doses of the stuff, than it is to buy different packages of the stuff in it's recommended size dosage packaging. I can say that it is a great marketing tool for the company that makes the stuff. It is also designed that we are less likely to get any of it on ourselves too.