100 Gb/s Ethernet is likely to be expensive, but dual-port 25 Gb/s Ethernet NICs are not much more expensive than dual-port 10 Gb/s NICs, so whenever you are not using the Ethernet ports already included by a motherboard it may be worthwhile to go to a higher speed than 10 Gb/s.

If you use dual-port NICs, you do not need a high-speed switch, which may be expensive, but you can connect directly the computers into a network, and configure them as either Ethernet bridges or IP routers.