P.S.: I'm still not sure why Dead Space costs €20 on GOG.
I can think of two reasons:
1. Because they want to maximise profit and they think not having DRM can be thought of as an premiere service, worth paying extra for.
2. Because they want to maximise profit and want every potential customer to pay as close to the max they would be willing to pay as possible, so they start out high and will then gradually lower the prize hoping that people will buy the game as soon as it passes their price ceiling (this probably has some fancy name in economics, but I am to lazy to seek it out right now). Why not start at the Steam level? Because they probably figure that some GOG users are unwilling to buy at DRM laden stores no matter the price, so some of them might still have a high willingness-to-pay despite not having bought the game elsewhere at a lower price point.
But perhaps the question was rhetorical.
EDIT: And for the record: no, I don't approve of this practice, I just suspect they reasoned along these lines.