That's right. I wrote some computer books some years back, and the royalties were somewhere around 10% of the wholesale cost of the book. So a book with a $40 shelf price might have a $20 wholesale price... and with only about 6000 copies as a typical print run, I didn't make much at all.

And also bear in mind that an advance is just that: it comes off the royalties, so you may not end up making any money for quite a while.