Lynn, it might not be that bad for your other cats - does only one have an existing health condition? My two are 13 and 14, and I pay just under £15 a month insurance, although that prob will go up at renewal time, as I managed to get a 10% discount on both of them!! Sainsbury's offer 5% for online booking, and 5% for second pets - excess is less than PetPlan too.
As for food - I am with Susanne, that a better quality dry is better than better quality wet, mine all have 2 wet meals a day, with restricted dry (except for fosters, who have it down all the time - if my two have dry down all the time, Lucy might eat it) - if you do a search for dry food and teeth on here, you will find some links that show dry food isn't always good for their teeth.