Again, if all else is equal, 10x50s will be more expensive than 7x50s. Let's look at these factors one at a time. This insures the brightest, highest contrast, and sharpest image possible. While optical quality is very important, moderate optical quality can be achieved inexpensively. Without proper construction and alignment, however, the resulting binoculars will be of poor quality.
Alignment is often poor in inexpensive binoculars initially, and deteriorates rapidly with use.
The most obvious effect of poor alignment is a double vision. A more subtle effect is severe headaches caused when you attempt to compensate for the misalignment with your eyes. Unfortunately, it may be hours after using the binoculars that you notice the effects. This is the single biggest reason to buy better binoculars. Quality of Design Design, both mechanical and optical, is the third key criteria of a good pair of binoculars.