It's not the 0.10 extra carat that you're getting, but the actual number...i.e. being able to say the stone is at least 0.50, 1, 1.5, etc. There will always be large increases at these milestones but it doesn't mean they may necessarily be worth it. A well cut 0.40 may look the same or larger as a 0.50 if the 0.50 is cut too deep. One good piece of advice on here frequently is to treat it like the price is right...in two stones with otherwise comparable specs, get as close as you can to your "target" weight without going over. That way you will still be able to get the same look and finger coverage without the added price inflation.
It also depends on the cut of the stone. A shallow 0.4 ct will look as big as a deep 0.5 ct. However, when comparing two very well cut stones, the difference in dimensions are 4.7 mm versus 5.1 mm. There will be a visible difference side by side. Why not find something over 0.45 ct without going over the 0.5 ct where you get the best of everything - you can call it a 0.5 ct stone and get a bigger stone without the price jump?