Is the 10g on WAN or LAN I think is the real question here.
I think both costs are relative. You could argue 2.5g doesn't make sense but it really depends on need.
If it's WAN then I'd argue both cases are expensive.
If it's LAN then maybe there is a minor cost difference between 2.5g and 10g.
Also just because the network is 10g doesn't mean your gonna get 10g across the network. If WAN ain't 10g not happening. If storage/NAS isn't SSD or SAS not happening. If the motherboard doesn't have enough bandwidth to support the 10g output then it's probably not happening.
Messy...just messy