We buy multi-packs of frontline for fleas (6 in a pack) which works out cheaper in the long term. Also, if they are kept flea free, they can have a cheaper (MUCH smaller so easier to get them to swallow) Worming tablet - Milo something rather than Drontal which is good but sooo big my cats won't take it!.
The last time we bought Frontline it was £30 for a pack of 6, and £15 for a pack of 3. Our vet does worming tablets free (but they're huge), or you can spend a fiver and get either a smaller tablet or a sachet of powder to put in her food.
frontline is very good and probably the best,but the last couple of times we've bought the bob martins flea stuff(that you also put on the nape of the neck)and it was so much cheaper at only about £5,or even a bit less.
we get it in sainsburys and use it on both our cats and they've never had a flea problem.