I saw this interesting product on Amazon while searching for "bulk Advantage" on Amazon.com
The idea is simple: buy a large dog's dose of Advantage and measure out the appropriate amount for your smaller dog since they are priced equally in terms of doses.
Interesting thing is, the active ingredient in Advantage I is 9.1% Imidacloprid.
You can buy exterminator-grade 21.4% Imidacloprid (used to kill termites for entire houses) in 27.5 oz (813 mL) quantities for ~$30 on Amazon:
21.4% is roughly twice as strong as 9.1% Advantage so you would need half the dosage. The dosage for 55lb+ dogs is 4 mL of Advantage so you would need a little less that 2.0mL (1.7 mL to be exact) of the 21.4% strength stuff. For 1 dog 55+ lb dog the entire 813mL bottle would give you 406 doses or 33.8 years of coverage if you applied it monthly. That's ~237 dog years.
The MSDS for this product shows that it is non irritating on contact to animals. The LD50 for skin contact is 5000mg/kg so there is little chance you could overdose your pet on this (unless you give it to them orally). It would also be fine for both cats on dogs because it is just imidacloprid.
Cats under 9 lbs use.17ml
Cats over 9 lbs use 0.34ml
Dogs 1-10 lbs use .17ml
Dogs 11-20 lbs .43 ml
Dogs 21-55 lbs use 1.0ml
Dogs 55+ lbs use 1.7mL
of Dominion 2L
Any DVMs want to share their opinion on this? I haven't tried this on my own dog but if you wish to do so you are proceeding at your own risk. I am only suggesting an idea and can in no way be held responsible for its consequences.
*Do not use any flea treatment that contains Permethrin on cats, as it is highly toxic and may result in death. This post is for Advantage, not any other brand.