I got the Frontline spray bottle from my vet about 3 weeks ago for my cats.
Since we have 4 cats (and 5 ferrets), he said that it would be more
economical to buy the spray on than the spot on.  The only draw back is that
it's more of a challenge to apply than the spot on.
 
He charged me $20 for the bottle, which covered 3 cats (cat #4 was just
spayed, so I'll spray her when the stitches come out) at 1 pump per pound
with about 75% of the bottle still full.  I think the spot on is about $30
for three applications.  My vet told me that I should have enough to do the
cats through the summer.
 
At 2 to 3 pumps per ferret, plus about 10 per cat, the 1 bottle should last
me 2 or 3 months.  That would come to 7 to 10 bucks a month to control fleas
on 9 animals.
 
Even if it lasted 1 month, it would be well worth it.  Twenty bucks a month
to control fleas on 9 animals is a deal I'd take any day.
 
As for efficacy, its been about 3 weeks since I first used it on my cats,
and there has been no sign of fleas on them!!  HOORAY!!!!  I wanted to be
absolutely positively sure that it was safe to use on my ferrets before I
sprayed them too.
 
Frontline is supposed to kill spiders, ticks, the ever dreaded flea, even
ear mites plus about 200 other nasties (according to my vet).
 
To Mary, thanks for some great info.  Did you ever consider becoming a
Statistician??  Thanks also to everyone else who replied.
 
FYI, I am in no was associated with the company that makes Frontline.
[Posted in FML issue 1921]