.. there are free ones. The post is only about the MH ones however.
And for $1 or $2 you are looking at custom folders, music, backgrounds and a bit more.. On my vita and PSP all I can really do is just add a picture to the background. (though it's been a bit since I looked)
Also the Themes (MH ones) are to match for the most part the two faceplates releasing for the regular N3Ds releasing in Japan and AUS(eu) this year and the US next year. So even though you can get them on the current 3ds' and the N3DsXL your paying not only for the background, music. ect. but for it to match the front of your N3Ds as well (if you get one).
So basically a full MH themed N3Ds... Complete with background music and if you buy the bundle from the US a lanyard to attach to it!