I think at the end of the day it really comes down to the donors. Where does the donors' interest lie, in which area of the charitable sector? Are they more interested in the environment and preserving the land they own for the future or are they more interested in making a donation to another area of the charitable sector?
If the developer were interested in acquiring the land, that's the fair market value, and presumably if the donor gave the land to a trust, she should get a tax receipt based upon what the land would be worth if it were sold to a developer.