Shelves. I sell or open things periodically when I'm running out of space.
Regarding money, I have a few techniques for helping the hobby pay for itself:
1. I try to make things free when possible. For example, if something is on 50% clearance, I could buy two of them and sell one for full retail price. The one I keep is effectively free. Likewise, if something is 25% off, I can buy 4, resell 3, and the one I keep is effectively free. This requires a lot of short-term storage because a set that is on clearance doesn't tend to start selling at full retail for a year or two.
2. I sell the figures and keep the parts if I buy a set in multiples. I'll generally keep the figures from the first copy of a set I open, but I'll sell the figures from any additional sets unless they're extremely generic and okay to have in multiples. (Example, I would only keep one king or queen, but I would stock up lots of generic soldiers or peasants.)
However, most of my MISB sets are not sitting around to make an eventual profit. Most of my MISB stuff was purchased because I want to build it or use it for parts someday, and I simply keep it sealed until I have the time and space for it. If I decide that probably won't happen or I lose interest in the item before I open it, I put it up for sale to free up space and money for something else.
There are also a lot of MISB sets in my apartment that I paid full price for, and won't be reselling whole or in part. I can't get a deal on everything, but I try to make the most of it when I do.