I don’t know but i would tempted to ask “why do you think it’s any of your business what I spend money on!” Flip it around and grill them about “their” problem.

Ok, maybe that’s going too far but seriously, once we get diagnosed with bipolar suddenly every seems to think that they have the right to scrutinize our every move. Probably it would be better better if you said something along the lines of “I appreciate your concern, but this decision to spend money is well thought out and is not an indication that I’m on a manic spending spree.” Be firm that you are manageing your own illness. You have every right to do that.