I am looking for an inexpensive (<$100), basic, software tool that can
outline, construct a graphical decision tree from the outline and let
the user assign probabilities and payoffs for each branch, and
calculate expected value and minimax solutions.  Something like what
MORE on the Mac used to do ten years ago but with numerical analysis
built in.

Does something like this exist that runs on Windows (preferibly) or
Linux (acceptable).

If you have Excel, you might want to look at TreePlan:

http://www.treeplan.com/treeplan.htm

Please take a look at this page. It has several links to the type of
software you need.
http://www.kdnuggets.com/software/classification-tree-rules.html

Thanks.