Budget Battle is a set of free wargames rules for fun games with plastic army men and their ilk. The author writes:
This set of rules was inspired by some sets of cheap toy soldiers with interesting pairings such as “Army men vs Cavemen”, “Ninja vs Robots” and so forth. These reminded me of the running joke in an episode of “Angel” that was about “Cavemen vs Spacemen”. One set was “Pirates vs Ninja”, reminding me of a side mission in Saints Row 2.
Now at last, the great philosophical debates of our time can be settled! These rules are not intended to be as detailed or as sophisticated as some rulesets. They are probably not perfect, just something I jotted down one weekend. Many of the game mechanics are also used in a variety of other games. Some forces will be stronger than others and it may take a few games to determine ratios that give more balance.
These are just intended for a bit of fun with some toy soldiers, a fist full of dice and some measuring sticks or a ruler. Adapt them and modify them as you desire.