Well, I'm definitely no expert on the subject. I think it depends who you are targeting with the book.
For someone who already played a bit with SGDK2 and wants to push to the next level or wants to implement some of the features you explain in the book, I think that 48$ is a totally acceptable price.
For complete newcomers who want to get started on SGDK2, I have the feeling 48$ is a bit much. All the way up to 30$, I would have no problem paying that much to try something out. Over than that, it seems to me that it may make the book a little less accessible.
Of course there is the fact that it's 260 pages with color and like I said, it's pure gold when you want to develop the features described in the book, which are common and very useful.
But, like I said, I have no idea what I talking about... This is purely based on feeling, so I wouldn't put to much credit in that.
I'm aware that going from 48$ to 30$ is quite a price drop, it might not even be possible.
Personally, I'm also taking into account the exchange rate. From USD to CAN, it raises the price quite a bit these times. It used to be a 1-to-1 exchange, but since the oil price drop the CAN dropped quite a bit. Today, the CAN stands at 1.34$ for 1 USD. Which means I would pay 65$ for a 48$ book, which is a little painful. But it's not a factor in the US of course...