I stopped using Frontline, because it was basically having no effect on Kai and Zev's fleas. I'm using Advocate now, which seems to work (when I remember to do it), but I think is prescription only, so you'll have to get a prescription from your vet, and send that off to an online supplier if you really want to drive the price down.
(Personally I'm happy to buy mine through my vet, but I appreciate that some people may need to get the costs down as much as possible).
Having said that, there doesn't seem to be much evidence for Frontline loosing it efficacy, other than anecdotal, so it's probably still effective in most cases, and since you can buy it in places like Sainsburys now, it's probably a lot cheaper.
The trick with keeping the fleas under control, is to regularly treat them, without fail, every month. Waiting until they're out of control will result in it taking a lot longer to get them back down to sane levels, and probably isn't particularly good for the mogs either.