There isn't anything that will get rid of fleas for good, though if you use a good product for some months you may not need to use it as often forever. Fleas, like all insects, are around in the environment and you'll never eliminate them all for good. I don't have to treat my animals for fleas very often any more, in fact I only have to use a flea preventative about once every 2 years these days, but then I don't live in a hot climate.
Take the dog to the vet and find out what the vet suggests. A local vet will know what's most effective in your area. Yes, it will cost more than what you're doing. Would you rather spend a bit more on something that accomplishes what you want, or less on something that doesn't?