Think about it like Pokémon cards; You can buy like the deck things they do AND mini-packs and the mini-packs have cards that aren’t in the main deck, and people buy them all the time 
I would happily pay an extra $20 for running animations and $20 for jumping animations over the original pack. You could even re-name the asset on the marketplace “Starter Park” and the rest as “Booster packs”.
Or you could cut out the comparison to Pokemon entirely and just release smaller packs of new animations for like $20 (Obviously not the animations in the main pack but the animations people are asking for!) - That’s a way around the $60 limit 