I generally order from 1800petmeds.com.
IMO, the flea collars are just not as effective - the main issue being that the farther you are from the collar, the less it works, so they end up with fleas on their feet, tails, etc. There are newer ones that may work better, but I have no experience with them and they are probably near the same price.
To help cut costs, I stretch the time between doses. I use a monthly variety, but apply it a max of every 6 weeks. In the winter, I'll go a little longer. The caveat is that my dog is mostly indoors.
Consider other factors. I'd say that fleas are generally just a nuisance unless there is a major infestation and/or compromised health. But using a preventative keeps them out of your house. Do you have a lot of carpet? Is the dog on the furniture?
Personally, I'm more strict about heart worm preventative. I think there should be a bit of a balance between a few flea bites and putting a pesticide on my dog's skin that soaks in and lasts for 4+ weeks.