Depending on how you defined affordable, you'll likely need to pay 3 times more than what you pay for prodiet. Just give you a heads up. LuQEE listed above some of the brand that you & your cat might like.

What is the reason you decided to changed from the old brand?

for starters, you can try search them in cat boarding shops, grooming shops and even small-moderate cat accessories shop.
You wont find them in typical supermarkets or franchise pet store chain.
so far, ive been feeding my cats vitapets/tesco/prodiet kibbles with mixture of blackwood/RC + canned wet food.
so far  still ok, exceptnow their obese but mine only DSH  

I have feed my cats with prodiet dry food a couple of times before. All I can say my cats have good bowel movement with it.  Whenever my cats have diarrhea, I feed them prodiet. It calm their digestive system right away.

Middle-grade cat food that is not really expensive that I know are Kiska & Visscay. Less colorful & slightly better ingredients. You might want to try them.