I rationalize it as supporting the company that is making game(s) I want to play.
Pricing isn't any different than the other Ultimates but the release discount is definitely not as good. (25% as opposed to the 40% for Udyr or 50% for Ezreal). So yea, for Udyr you spent $15, Ezreal $12.50 and now Sona will be $19 on release.
I like the game and I like the content Riot creates and I want them to keep creating it. They give "free" stuff all of the time and they have become a HUGE company. $19 for a once-a-year skin? no problem! I was subscribed to EQ for 5 years which was ~$10-$12 a month?
The main issue is with people that can't make mature financial decisions. All gaming companies with FTP or P2W models will attract some of these people whom are addicted and need to drop every last cent into the game. On the flip side you have the absolute die-hard FTP players that make it a mission never to drop a single cent. Both types of people should probably not be playing the game. The addictive spenders probably have something akin to a gambling addiction whereas the FTP players should probably work at furthering their careers if they feel so cash-strapped. However, I do give leeway to the FTP players. Financial situations differ and someday maybe they'll decide to support the company once their situation improves (and they deem the content worthy of a purchase).
If you consider yourself a responsible mature person then the asking price for DJ Sona should not bother you. You'll either decide it's worth or not and leave it at that. You understand that Riot is making decisions to stay successful, as any good company should, and you make decisions based on your current financial situation, as any mature adult does.