Flea Treatment for multiple cats

I have 3 cats, and a kitten im fostering. My roommate also has a cat and my sister (Who I live with) has one. My sister's cat is severly allergic to fleas and a week ago, had nasty scabs and sores round her neck from fleas, and scratching. We brought her to the vet who gave her a shot of steroids, and she was treated with Revolution.
I didn't think my other cats had fleas, never seen any and I never seen thm scratch. Only my dog scratches but he goes outside. But I found a few fleas on Salem yesterday and now I know I have to treat all of them for fleas. (My sister's cat Peaches stays up in my sister's room, which is like the attic, its a loft, so Peaches is seperated from the other cats)
Since I have my 3 cats and the kitten to treat, ANd my dog (my roommate will be responsible for her own cat) I dont know what to treat them with. I want to get revolution but it would cost way too much. Over 100 dollars for a month. I wont use walmart or oter grocery brand flea treatments because I wont risk something happening to them from chemicals. Any ideas??

Oh I soooooo no what you are going through!!lol I have 5 of my own cats, plus 4 fosters!I can totally relate to how EXPENSIVE it is to get a flea treatmentfor them.I have not been able to find anything else that is as good (and safe) that is much cheaper!!I use revolution on mine and it cost me 89 bucks a month!I am not sure if it works as wel as Revolution but advantage Multi 6pck cost 72.99 (it treats worms,ticks,fleas and ear mites) or regular advantage cost about 55.00(also a 6pck) and frontline is about 56.00! this off the internet...prices may be slightly different!Good luck or you can use the frontline sparay ( which works on dogs,cats & kittens) and that is about 27.00 for a 250ml bottle!

I also have 9 cats in my house and 2 are highly allergic to flea bites. They also get hot spots and they scratch their fur off. I get Frontline from the Vet, I get 6 and they throw in one for free for $84.00. I couldn't afford that either for all of them so what I do is, I put front line on the 2 thats allergic to them and I make sure I fine tooth comb the rest of them. As I get the fleas off, I keep rinsing the fine tooth come in water with dishwashing liquid to kill the fleas. I know its not the best way to deal with the fleas but it does help. Good luck.