As far as I understand, mid-range ukulele would cost around $200-$300, as the beginner range is around $40-$150. The difference between ukuleles in different price ranges is basically about the material -- tonewood, and construction types.
For example, koa has probably been known as the best tonewood for ukulele and costs around $500 or up. Solid wood ukulele costs much more than laminate wood. And I think mid-range ukuleles are probably made with solid wood rather than laminate.
Also, the size of ukulele has to do with the price as well. A soprano ukulele will always cost less than a tenor with the same wood and construction (of the same brand/model).
Hope this helps.