Europe cruise season is May to Sept, similar to Alaska season

But into late Sept sometimes even early Oct depends on itineraries.

Most length 10 to 12 days - for an inside cabin, Per Person roughly $100 to $120 a day, before the $10 a day tip, and any incidentals (drinks, specialty restaurants, etc). Land excursions probably run you the same amount or more.

Even if you dont plan to spend extra days pre- cruise, dont fly in the same day to board the ship. Always fly in one day before - to make room for any possible problem in flight connection etc. Missing the ship is not a joking matter, trust me.

Making your own Air arrangement usually is cheaper than Cruise Air, (purchase air from cruiseline).
FF miles become very valuable for this purpose.