I don't use RR spells to upgrade things like the Festival Merchant (seeds), mana sawmill (mana), (and any other thing that shows up in crafting) since replacing them with the current version is cheaper than using RR spells.. With things like mana huts, though which are rare, I use RR spells to keep them at my current level. My strategy is to craft the things I can when they show up in the crafter and replace them in my city, saving the RR spells for those things which I can't craft but which are good producers.
AJ