[CaliforniaKarl]

Yes, 100G multimode transceivers are cheaper, but they don't use the same fiber.

100G on singlemode (100G-LR4 being the most common) uses the well-known two-strand ("duplex") fiber. Or you can get 100G bi-directional ("BiDi") over a single strand of singlemode (fiber-to-the-home often uses this).

100G on multimode is weird. As the name implies, one beam of light, sent down the core of a multimode fiber, results in multiple modes (search "Laser modes") being sent down the strand. As they overlap, it gets hard to get a clean signal out the other end.

To deal with this issue, 100G on multimode uses fiber cables containing multiple strands per direction of travel. MPO-8 and MPO-12 are common cables used for 100G multimode: It contains eight or twelve strands of fiber. Four strands are used to send, four to receive. And the prices for those cables are higher than standard duplex singlemode cable.