I did some research on eBay and on this group regarding whether you
can use the dog version of Frontline Plus on cats. I am not a vet, but
the messages on this group seem to indicate that they are
interchangeable. This is attractive because 12-month supplies (12
pipettes) on eBay all run about $85, irrespective of dose. I found the
following amounts are contained in each pipette:

cat   (all sizes)         0.50 ml
small dog (0-22 lbs)      0.67 ml
medium dog (23-44 lbs)    1.34 ml
large dog (44-88 lbs)     2.68 ml
X-large dog (89-132 lbs)  4.02 ml

Active ingredient concentrations:
  fipronil 100 g/L  (dogs and cats)
  S-methoprene 120 g/L (cats) 90 g/L (dogs)

so using the dog version on cats will give a slight underdose of the
second ingredient, which is safe. There is no reason to use the cat
version on dogs, unless you like to waste money.

The trick, of course, is in applying the proper dose to a cat. I just
bought on eBay a 12-pack of the X-large dog dose, which will be a
two-year supply for our four cats. I plan on dumping one pipette out
into a small bottle and then drawing 0.5 ml with a graduated
eyedropper from the bottle for each of our 4 cats. Each pipette will
be good for two months.

If you do this, please be sure to measure very, very carefully.