Just tested this myself and got the following results, bear in mind I only have base game and didn't myself complete all of these in my challenge, I also haven't done fishing yet but I'm guessing it will be similar to gardening.
Anyway these are the amounts I got for completing collections via debug.
Frogs + 500
My Sims + 100
Metals + 100
Crystals + 100
Elements + 100
Postcards + 100
Fossils + 100
Microscope Prints + 100
Space Prints + 100
Aliens + 100
Space Rocks + 100
Eggs + 100
Skulls + 100
Plushies + 150
Posters + 500
Gardening + 500
But here is the real kicker Magic Beans + 5000
Assuming for now fishing is the same as gardening I would not have earnt enough from collections alone for another knight, but added to exisitng funds I can't say for certain that I couldn't afford one because of it.
Only thing I can think of is if the amounts for all collections were worked out (I don't know what else there is) then we could remove that from funds before buying knights maybe.
I shall send my sims off fishing now to check that one out.
EDIT: Well it took a while, stupid anglerfish
but fishing is same as gardening with + 500
I also loaded up my final challenge save, I had more simoleans left than the amount I would have made from collections, but not enough to finish another one by buying knights so my score hasn't really changed, which makes me feel better.