Very nice, love this idea.
A couple of things that I think that can be improved.
1. Coloring the upgrades that can be afforded would be nice. I have to drag mouse over all of them to make sure I can afford them.
2. I have to do a lot of maneuvering with the upgrade screen because a lot of the time the price was just outside the window.
3. It seemed like there was a dramatic increase from the < $100 skills straight into the > $1000 skills, that stopped me from playing cause I didn't want to grind so much for incremental skills.