Deselect Enable on that variable's row. That is the documented way to hold it fixed for the run.
Before a probability simulation runs, DF prints a table naming every variable the random generator can act on, with a switch on each. Land acquisition costs sit as their own numbered profile, ahead of construction costs and end sale values.
The mistake worth avoiding is leaving a variable enabled with a variation range nobody believes. It does not fail loudly. It feeds noise into the simulated margin and internal rate of return, and the output looks like a result rather than like the assumption it is.