At our hospital we go out of our way to keep product affordable for the average pet owner with just a few pets. Generally we beat the prices of mail order places and without shipping costs, and for products like Frontline Plus we don't require an office visit (I'm told some places do that). My point being, it might be worth calling around to other vets in your area for prices on flea products. I'm sure we can't be the only hospital with this practice of making things affordable.

The best suggestion I can give personally is; try speaking with local humane societies and rescue groups, they may be able to help in the way of free or discounted products. Or possibly a few local vets may be willing to make a donation of product/services or discount on product/services to help you with your cause (same also for spay/neuters). Sometimes writing a letter to the manufacturers that make pet foods and flea products can sometimes result in a discount or donation of some kind. A donation jar set up at local stores (with their permission) may also be a possiblity.

I applaud your efforts in caring for and reducing the breeding of feral cat colonies and strays!!

As to specific flea products...

We carry Program, Revolution and Frontline Plus at our hospital. The boss prefers Frontline Plus for these reasons, with a few exceptions:

Program does not actually kill the adult fleas only inhibits growth of the flea population, and only at the egg stage, and does not kill ticks. (If someone is simply wanting to prevent fleas and is not having a current flea/tick problem or had one in the past, my boss does recommend this one occasionally.)

Revolution also inhibits the growth of the flea population, but also actually kills adult fleas, but does not kill ticks. Revolution is more expensive (at least at our hospital) than using a heartworm preventative like Tri Heart Plus in combination with Frontline Plus for dogs. (Although for cats outside or with ear mite problems or for dogs with mange or ear mite problems, the boss tends to prefer Revolution.)

Frontline Plus not only inhibits growth of the flea population, but also actually kills adult fleas, and kills ticks. After considering the EPA's stance on both ingredients the boss feels that Frontline Plus with Fipronil and Methoprene to be a safer choice for both pet and human. (This is also the product I use on all 5 of my cats as they are occasionally exposed to ticks)

I do know is that there are imitation products out there that are not manufactured to the US standards and without EPA approval. I also know frequently these products look very much like our US versions in packaging, but the actual ingredients may or may not be what the label says. The only way I know of to be sure is buy from a licensed vet in the US, beyond that I don't know.