Interesting, but you are comparing places that are far from North America with places in North America, so the airfare will be more significant for places in Europe.
First of all, would you plan to stay the entire two weeks in one of these cities? Probably not, since you could "do" most of these places in a week or less.
I suggest that you narrow down your planning to three places, then add one or two nearby places for a two week trip.
For example Barcelona add Madrid and perhaps Seville, Spain.
Prague, add Munich and Vienna.
Then research what you want to see. Check out TripAdvisor "things to do" and determine how long you wish to spend in a place.
Then using kayak.com, TripAdvisor and Booking.com to find the lodging that you prefer that suits your budget.
If you take day tours, price those as well and add cost of admissions.
If those numbers represent the cost for one person, then, I can tell you that you can find more affordable lodgings than the price represents.
Also, consider taking a guided tour that would include your lodging, fees, most meals, etc.
We love Gate 1 Tours that are a great value with reasonable prices.
Example, my wife and I did a two week tour of Egypt in January that included 2 three day river cruises on the Nile, hotels were five star, most meals, admissions and fantastic guides for $3500 per person. That price also included airfare from JFK NY to Cairo.
If you wanted to do a city like Washington, DC the main sites you can do easily on your own. Further, all the great museums there to see. Still, you wouldn't need two weeks for that.
Enjoy your trip