*cough* pure speculation *cough* is that you will need a grand total of 3600 DNA to upgrade the opera house in Act 3 to Level 9, which may or may not be a good goal to set, but that's mine for now.
That's just for the upgrades of the opera house. DNA is also used for at least two of the craftables in that stage, and you will also need corn for some of the craft items, especially the one I want the most, that I can't name here because it is pure speculation.
Fertilizer will come into play for only one of the craftable items in Act 3, from what I can see from the pure speculation, and for that item you need 4500 fertilizer, though I do not know if it is an item you can craft only once.
As for me, I'm hanging onto as much DNA and corn as I can for now, and if it turns out I can get some use out of upgrading Monsarno any further than I already have, I'll do it then. You'll be using about 2000 fertilizer for every level of Monsarno you upgrade, and I can't imagine that you'll lose much if you wait a few days until the start of Act 3, to decide to upgrade Monsarno. But that's just my opinion. :)
I'm just hanging on to everything for now, and deciding what to do when the time comes.
