There's a Universal Economic Principle at work here. When negotiating the amount of funds available for your use to construct, repair, enhance, furnish, equip, and/or fumigate your studio, it's easy to overlook the downside.
Whatever you spend on your studio, she will get to spend at between 10 to 100 times that amount on whatever. In addition, your expenditure is a one time event, whereas she will run a lifetime tab. Furthermore, during any future conversation, on any subject, she is free to make the point that she allowed you to waste all that money on your studio.
It's an inescapable fact.