It's always hard when nothing was wrong with the old item and continuing to use it would have been just fine. But you mentioned that you've wanted this new monitor for awhile, so I'd say the fact that you were willing to wait (even though the post title says 'impulse buy'), shows that you were using your frugality muscle. I think people get themselves into trouble when they impulse purchase things they didn't even know they wanted before.
Honestly, some car payments are $500 a month. That doesn't mean you spend that much on whatever you want every month, but I'm pretty sure if you really want it and are getting great value out of it, there's nothing wrong with treating yourself every now and then. If you buy it and never use it, well, that's usually a different story. And as the others have said, it really depends on your income and your savings rate. I'm happy at a 50% savings rate, but I still find I torture myself when I spend the rest, only to pull myself back to the reality that what I spent was a fraction of what I saved that month. Perspective is key.