100 GbE isn't that expensive. Transceivers for 100 GbE are in the $800 per port or so. CDW sells Proline 100 GbE transceivers for $300 a pop. It's a standard QSFP28 interface that is used for most of them. It carries 4x 25 GbE across the fibres.

They also make 100 GbE DAC cables.

Longer distances do cost more... Just looking around I found some 100GBASE-LR4 transceivers going for $6k a pop from Finisair... but at least you get to continue using your single mode fiber ;-)