My vet told me to feed my cat Urinary SO and I have some questions?

This week James wasn't looks too good. We took him to the vet (prepared to have him put down) and they said they could help him. After ALMOST $400, he was catheterized, drained and we were given a bag of Urinary SO from Medi Cal (and or Royal Canin) and they told us to ONLY give him that food and that they never get another cat in with crystals again. The biggest bag they have there is around $110 (for around 5.5 months for just one cat) and they want me to put all three cats on it. Just the male cat was ordered on it. I'm not sure if it's a good idea or not. When I asked them about this other one called Fish and Potato with Blueberries from First Mate, they said to NEVER give him that one and that if I stray from the Urinary SO diet, the crystals WILL come back. Just looking for some guidance on this. I live in Ontario Canada and hoping there is something out there as good or better. I did start him off on the canned food and he likes that. He's not too happy with the bag.