[graeme]

An app is more like a capital expenditure. You pay once, and never again.

You definitely should think twice, but the $400 monthly figure seems like the wrong comparison. You likely spend most of that on monthly expenses that recur.

I have the same issue of a limited budget. But for some purchases (software, books, furniture, etc.) I consider them on the basis of "does this provide X$ of value to me?"

If the item does provide enough value, then I use money I've set aside for that purpose, and I buy it.

Setting money aside for this sort of one-off value purchase is part of my monthly budget.

(Of course, I also have to weigh a $50 app purchase against other uses for my saved money.)

edited to add: I'm not saying you should be buying $50 apps of course, that is a lot of money on your budget. I'm just saying the reference point should be long term rather than monthly.