Hey,

its really to expensive to rebuild and level up mines...
why not make mines work like the stones and marble pits instead ?

if you want it to cost something to gain a new mine, make it so, that it costs resources to send out the scout to find a ore mine and that way make the upgrade on the building that collects the ores and not the mine it selves.

hope you understand what i mean about this.

regards
Jesper