If that's the case and there's no way to do a lookup with an ingame constant, function or both i'll probably just hard code in the upgrade costs, and work it out that way, cheaper and easier, until such time the API provides a means.
Aside: The game has to read the max bag size and cost from somewhere though, i feel i'm missing something. Though i guess certain data is outside the scope of the API and thus unavailable.
That's pretty much all i could find to Garkin, i wasn't sure if i was looking hard and deep enough or whether i was simply missing something totally obvious to someone more familiar with the code base.
Thank you both for having a dig around for me, unfortunately it only confirms my own conclusions but it's nice to have some confirmation all the same.
I'll stick to pulling the values from the wiki and hard-coding them in for now, it'll mean adding or tweaking items in an array any time these change in-game, but i think i can live with that.