I would say lightheartedly, "Well, we/I think it's worth it. Bean dip?" If they try to engage further, such as, "But don't you realize the cost?", I would say, "Yes, you've mentioned that" with optional flavors of bean dip.
If they bring it up constantly, I think that would be a time to take one of them aside and say, "Look, when you remark on my purchases like that, I feel uncomfortable." I personally, if the friend were close enough to me, would add, "My finances are none of your business unless I involve your money or property in it", but the friend sounds a little sensitive, so that would probably come off too harshly.