Insulate it with some fiberglass, Kraft faced paper insulation. This is the least expensive route.
Just make sure the paper goes toward the heated space in your climate. This means you should not see the paper when you insulate from underneath. The paper will be up against the bottom of the floor boards. Use wire insulation stantions to hold it in place if you need them.
Then put some plywood underneath, no plastic, just to keep out the critters.
If you are in a fire prone area, you may need to use a fire resistant material. I have used durarock in the past , but there may be similar materials in your area.