It is interesting that your assessed value dropped that much in Urbana as our local market has held up very well in the past 2-3 years. There is one development in particular in North Urbana that has had a very rough time and we've seen the values fall pretty quickly.
In theory your property value has dropped but tax assessors (love them as people) are notoriously wrong in their estimations of market value. The down side to their job is they are asked to take an exceptionally dynamic marketplace and boil it down into a linear formula. The problem is that that just doesn't work. There will always be expceptions and in something as dynamic as real estate they will be more common than not. The good news is that your taxes are lower in the short term. Keep in mind that you haven't lost any money until you actually sell. If you are staying put for a while, you're fine.