We are trying to figure it out as we just played the first day and can "modify" a few items since the US did not see any of the upper levels (taking over the maggot valley).
If you can trade fortified buildings for 8 each I would buy 2.5 of them for each FPP purchased tunnel directly.
This is the same problem that RB had for tunnels. If you could trade a fortified location (cost 10 FPP) for a tunnel, you would never buy a tunnel for 20 FPP unless the rules (in this case the FPP chart) basically implied that you can only get a tunnel for the higher cost.
There is no specific rule saying you cannot exchange fortified locations (cost of 8) for a tunnel, but it is implied.
If exchange is allowed it would change the purchases dramatically as tunnels would be "cheap" enough to buy en masse.