I am a cheap SOB and have spent, literally, two or three years planning this trip. In general, you should be able to book most cruises for $50-$100 per person per night for an inside room. The only exception is if you are on one of the luxury cruise lines and they are just extremely expensive. Like $500-$1000 per night. If you have lots of free time transatlantic repositioning cruises can be a great deal for about $40 per night. That said, since this is a special occasion, we wanted to have a balcony room which of course comes at a premium. Also we wanted a certain schedule to see as many countries as possible. These days you can spend more on airfare to get to europe than the cruise costs. Our three legged airfare is around $1100 per person (ridiculous!) and the cruise was $1599 per person. We were able to book an outside cabin and get a guaranteed balcony upgrade when they had a sale. Here is a link to the website. Click on the map link to see the route.

http://www.ncl.com/cruise-ship/spirit/vacations#tab_detail

When planning a trip like this keep in mind how many days you are "at sea" because it is hard to tour a city at sea! Also remember that they expect you to tip daily and that alcohol is not included and can be very expensive on board. We booked on NCL which we kind of like. We also like Holland America a lot. Princess and Celebrity are also good medium priced cruise lines.