I use 
REVOLUTION and pay 112$ for a year's supply (once a month application) 27-28$ for a 3 month box times 4 times a year! No prescription needed if ordering from Canada or Australia ... itz a vial application ... not a pill!  
http://www.canadavet.com/revolution.html 
this website might be cheaper ... 
www.petsupplyexpress.com/browse.htm 
these places also sell the stuff you use .... check it out!