Keep in mind that river cruises (in Europe) include alcoholic beverages and shore tours in the price of the cruise.
Compared to large ships cruising in Europe I don't see that much difference, and what there is is worth it.
These little boats get you up close to the sights 24-hours per day. The food is excellent and when it comes to immersion in the local culture you won't find a better cruise.
I see river cruises starting at about $1800 per person. Compared to a big ship cruise where you add in tours (as you will certainly want to do in Europe) and wine with dinner, I think they are good value.