Afforess
The White Wizard
If we extrapolate from the actual cost of the Eurotunnel, the 31 mile tunnel that connects France to the UK, the final cost of which was around $12 billion or $387 million per mile, the cost per mile is roughly 100 times the cost of paved highway. 500 gold per tile is not excessive considering the strategic value of connecting two land masses.
Hmm. You have a point. I'll update their cost accordingly.
Also, as in real life, tunnels should be difficult to destroy. Maybe needing plenty of torpedos (subs) or nuclear weapons to do so. The high replacement cost will entice the owner to defend them and the enemy to destroy them, making for an interesting cat and mouse side game. If you want a cheaper version of the tunnels, maybe you could add bridges, which though cheaper to build, would be much more vulnerable (planes, ships, etc).
I'm not sure I agree. Tunnels being tunnels, shouldn't they be rather to easy destroy? A small explosion which ruptures the pressure balance would rip it apart. I think players should just try to defend them.