$10 is a bit cheap really. That's pretty much the same price I pay for my lunch a few days a week. I'd say $15-$20 is reasonable for any budget game, which is about on par with a cinema ticket.
I think $30 really isn't bad seeing as it's a long game. Cheaper games like Peggle tend to have no story and pretty much just have arcade-like gameplay, which Aquaria doesn't. I think Zaphod has a very good point with bringing up the perception of value: what exactly is it that makes Aquaria seem so undeserving of less than half the amount of money as the many shit Xbox 360 and PS3 games out there? If it delivers relatively high production values and a memorable story-based experience, is it really not worth half the price of a retail game?
I think it is.