Simple answer is find out the difference per board ft for the wood in
question, and then it is very simple to figure out how many board ft
will give you a savings of the cost of the 2 machines. Forget labor,
but figure in the cost for new blades or resharpening, and that is
going to be hard to guess at precisely, but for simplicity, figure on
replacing the blades at least once

If you can save 50cents a board ft buying the rough sawn lumber, and
you spent $800 for the jointer and planer, then you need to run
something like 1700-1800 board ft thru to break even

Plug in your numbers (cost of the tools you buy, cost of blades, and
the cost difference of the rought versus the finished lumber

John