I myself space these purchases out. I'm currently on a long term project to go through all the campaigns with all the characters except for possibly Kuzman's with some characters. When I start a new character, I buy the skins. (Ginebara might be the exception because the Slave Unit is still somewhat ridiculous.)
While they are kinda overpriced, I justify this to myself three ways.
1) I honestly don't spend a whole lot on gaming. I almost didn't get an X-box One in favor of quitting new gaming altogether. Netherrealm Studios and my bi-weekly XBL sessions with a friend changed that but I spend very little on gaming. As such, I can better justify the purchase.
2) Supporting the Devs.
3) I played Marvel Avengers Alliance and, when that game closed up shop, everything I spent money on vanished. I play SWTOR and, when that game closes up shop, I will also have nothing to show for it. It is the nature of the current state of gaming. If it's online, you don't truly own it.
With this game, even if I were to buy all the skins, I still wouldn't equal what I've spent on the other two individually plus I get the skins to keep.
These are my reasons. They do not apply to all but they are mine.