If you play other smartphone music games, this comes out to less per song than most of them, off the top of my head.
The way I see it, assuming the game cost more and came with all the songs available, I wouldn't see casual fans playing all 30 songs or so between IAP and included songs. I think the IAP is for more dedicated fans who want to practice more or just try more songs, and for slightly spendy casuals who would pay to play a handful of their favorite songs. I've been playing music games for a while and game I'm playing the most right now has about 1100 songs with usually 6 charts each, I don't think most people who would just play a handful of credits rather than playing regularly would make a dent in playing every song, much less every chart.
I guess SNKP could just release a demo with one song only, but I would imagine most people who would try the demo would also pay $1 for a generous assortment of songs. I think they've struck a good balance.