Fiber isn't really all that much extra anymore. Duplex single-mode fiber is like 1/3rd the cost per meter as Cat6a. It's more expensive at the margins (optics, switches) but if you want to go 10gb, it ends up being a wash. And fiber will get you up to 400G+ if you ever want it without new cables.
The biggest cost saving in fiber is in the lightning strike case. Assuming the power lines are properly grounded and surge arrestors appropriately installed, at least a strike won't fry your expensive switch or even more of your infrastructure.
At ~$80 for 2x 20km 1000BASE-BX BiDi transceiver and 2x media converters it honestly comes down to the cost of the cabling even for just 1 gig, and for that you just need a single strand of fiber so the cabling can be even cheaper.
