Unfortunately without access to the MLS it can be quit difficult to find land values. The Cook County tax records can be a good start, but not necessarily the best source. The variables are endless. Lot size, location and Zoning are factors. In areas like Lincoln park, there is a premium charge for more then one consecutive lot being sold as a package, or a lot on the 1900 Block of Burling St. is going to go for more then the same size lot on Dayton Street.
The reason the Cook County Tax records will not be the best source in the City of Chicago, is the vast majority of properties that are being sold as land have a pre-existing structure that needs to be torn down. Therefore the Tax records will not have it listed as vacant land, making it very difficult to find the comparisons.
Your best source is a Realtor with access to the MLS.