Without knowing a lot more and ideally seeing the buildings its impossible to put a value on them.



There is such a huge variance in building designs, construction quality and previous maintenance that they could be complete junk ready for the excavator or they could be in like new condition.



A 10 yr old quality built and well maintained building will sell for more than 50% of new.    For 1 it is sitting on land, its got the utilities hooked up, the permitting is done and it likely comes with a pig flow.    
If you've never built a new building, it is a headache prior to and while being built.    Then when its done and ready the migraines show up.



If a building is properly built and well maintained it can last a lot longer than 20 yrs but it does take more maintenance as it ages.