I currently have 38 fields and the 39th asks for 750 catnips. Are they still worth the food I should spend for them? To what extent? (that is, how many catnip fields can I have before the price makes them worthless) I currently have 12 kittens
Catnip fields give infinite catnip. Therefor more fields is better. At least, that is my philosophy. I have 47 fields right now, and the next costs 2057. During spring I have no farmers and +1.5 catnip. During summer and autumn I need 1 or 2 farmers, depending on the amount of catnip I have to survive winter. During winter, I have most of the time negative catnip production, but I have a buffer (that was a tip from a previous question) so that does not matter very much. My buffer is now 3500, after a warm summer.
It's not mathematical my answer, but I try to upgrade my field when my buffer is more than twice as much as the price of the new field. In that way I still can survive winter and have as least as kittens possible on the farm.
If you want to calculate if a new Catnip field is worth the investment use:
Price/ProductionRate = Seconds Till Profit
Example, if a new field costs 10K
10000/0.63 = 15873 seconds
Convert seconds to hours
15873/3600 = 4.4 Hours
A field that costs 10K will start paying you back in about 4.4 hours
Of course the modifiers and seasons change this payback rate, generally the payoff time will be much better, this simple math can help provide a baseline.
I mean, you should always buy more fields. Once you have hit the storage limit, THEN start converting to wood. More fields = more kittens you can support, AND more wood you can convert, in the long run.
I would advise buying as many fields as you have patience for before buying a single hut.
(source: have started from scratch in a new browser multiple times, have a good grasp of early game)