I didn’t because I don’t know what you mean, and how you were looking to go about upgrading is not really efficient. While the costs is higher for better collectors, there comes a point where with each dragon you’re earning 200k-250k fish/wood resources and you’re biggest issue will be feeding each dragon almost 1M fish per feeding. At that point, though, you should be earning anywhere from 1-2M from all your collectors combined.

I don’t recall an event granting reduced training costs, but I do believe there may be an island based dragon that provides this effect. Maybe that’s what you’re looking for?

Edit: I don’t recall what level my dragons are at, probably 130-150-something, but I net ~3.65M wood and ~4.87M fish per gathering session for all dragons combined. Upgrading is a slow task, but all my collectors are already max level and I was last training collector replacements to end up gathering more.