Building on the "price reference" for others discussion...
$350, all in, including air, is a great nightly rate, I think. We went in early August of 2015 and it came to $410/night for a 7 day. Do the per-night prices drop for longer cruises? That would be easy to check, but I haven't done it. And I have no clue if prices for river cruises have been pushed up or down between 2015 and 2017, or how currency valuations have changed, in that time etc. For me, though, river cruises aren't worth it; once you net out the differences, it's 3 times more for a river cruise as compared to a "good port selection" large vessel ocean cruise.