The i850 is twice as fast as the smaller i250, and three times as expensive. On the other hand, the output measured - six pages per minute - for the most economical of the Canon printers will satisfy most people.
Color output ranges mostly between three and four pages a minute. Only the i550, and especially the i850, really stand out from the crowd in this respect. The i850 remains the fastest personal inkjet printer. It is even more amazing when you realize that the average color speed of the 80 inkjet printers tested in four years is 3.1 ppm.
To summarize, if this quality is important to you, the price increase is justified by the increase in speed. Except in the case of the i470D, which is more expensive because of the card readers.