I paid 5mil eco bal for my last empty Isle non expanded or built on mapart usable island. Then it just depends on if you want to pay for it to be expanded enough(one expansion may not be enough, for me it was.) And factor in the cost of each expansion. In comparison, I sold my original mapart island to ruwi for around 20mil fully expanded and fully built/stocked with hopper sorter etc.
In my opinion getting one yourself is easier and quicker than getting the grass/bal for it. I've been told by staff you don't get punished for resetting islands over and over as long as you don't collect any of the resources in it.
I'll sell my current mapart isle for $35m xd I have a double chain Isle on survival that could both be expanded and used I think
Idk if yall realised but this thread is under the "skyblock" tab, rather than eco sooo... mapart islands on sb go for around 20-54 stacks depending on demand and design.
