There are actually mods under development with this exact feature - underground tunnels. Its a Vietnam-mod which seems to be the first one exploring this possibility...
A screenshot of it:
http://eod.bf42.com/images/models/tunnel4.jpgAnd back to topic. I think this expansion is all but expensive, as someone mentioned below, the cost for each map is almost marginal. And thoose maps are made by professionals, who have had very big recources when it comes to tweaking and modeling.
Was 12000+ players online last night, the community is growing at a very rapid rate at the moment. Not strange though, since BF1942 is imho the best (most fun and versitile) teambased-FPS to date. Bring it on.