I would suggest that if your cat caught a bug and presumably was given some antibiotics as well as being on a drip, she should soon be back to normal. If thats the case and she was ok on her normal food before, I can see no reason for continuing on the food from the vet once the current lot has run out. However, if the illness was due to some sort of internal problem rather than something she caught, then you might need to ask the vet what sort of 'bug' it was, and if you explain about the cost of the food they might be able to suggest another type. You can get special food for certain conditions from the online vet pharmacies which are cheaper than the ones the vet sells, such as foods for cats with kidney problems, stones etc.
If you know the condition, let us know and we can have a look for you. Hope she is soon back to normal.