I do have that kind of rationalization at times. At least it sounds similar, but substantively, it's not the same.
I decide I'm going to buy X. I know I can fit it into my budget reasonably, even if it's a stretch or I know I'll need to be extra good the a month or two afterward.
Before I buy X (or within the return window), I see Y. It may be slightly higher, or lower, or about the same. And I reconsider X. Which do I want/need/can use more? I decide on Y.
Is this rationalization? I don't think it is. Because the money for the purchase was already there and set aside in my wallet, budget, or mind. Changing my mind about what I spend it on isn't any kind of specious reasoning or justification of a splurge.
The real issue is if I turn down X in favor of Y, and then forego Y for Z which costs is more than both of them combined. That's like April's original example, where I'd be counting the same pot of money multiple times.