Veterinary question: Can I cut Comfortis in half?

Hi! I have two dogs...both about 50 pounds. I would like to know if I can cut the higher dose Comfortis tablets in half with safe and effective results, rather than paying $15 each for two that are for dogs around 50 pounds, can I get away with buying one $15 tab for 100lbs dogs? I feel awkward about asking my vet...like I'm screwing her out of sales...but if I can pay $200 a year vs $400 a year for flea medication that'd be good!