I think 15 is a well-rounded price, sure it's missing quite a bit, all the create-a-_______, few levels, few characters.
Seven levels is still a good amount, truly, I wish there was more, probably like the rest of you, but seven isn't bad.
I think a batch of THPS3 levels isn't the only dlc that they will be thinking of adding either, so we can expect more and more levels, granted, it's dropping more bucks just to play them. They will be adding characters with each DLC as well, so far, all I've heard about is steve caballero.
My hope is that if this sells well enough, they'll consider developing a full on game, on disc, that includes aspects from each game in the series, excluding ride and shred, of course. It's a bit of stretch to assume that, i'm aware, but i'd like to support them by buying this game just on that alone.