Prescription diet

I took cocoa to the vets yesterday and was told she is overweight on the scales she was 24 kilo she was spayed in August and was 19 kilo I was told she would put weight on but was surprised by how much . At the time they didn't tell me that after the operation she doesn't need as much food so I have been feeding her as normal and yes a few treats as we'll.
They advised me to put her on a prescription diet and have to take her back every month to be weighed, the diet is hills metabolic wet and dry mixed.
My question is this tho is there a cheaper alternative to the wet food it's about £2.20 a tin and she has one tin a day the mixer isn't too bad £25 for 4 kilo and she has 100grams a day
Any advice greatly received

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