Hell, for that price 'Id just dig a big hole in the ground, and put sticks and leaves over it!
But seriously, the Idea of mines and boobytraps in the airsoft field is a very interesting and intriguing subject. It may not be practical with just one or two little foot mines because with the vast size of the field and the limited amount of players, someones' chances of actually stepping on it are pretty slim. But, it could be used in a type of scenerio where you already know someone is going to be walking.
Tripwire devices are also good too, because the wire encompases a greater area that one can walk into, rather than just happening to step on top of the device.
Id actually like to set up a "minefield" type of game where the whole field has oodles of hidden, burried and wired traps that anyone at anytine could unknowingly set off. For one thing, it would really keep you on your toes!
Ive actually made some boobytraps for the 40mm grenades. They use a hollow cylinder closed at one end for a body, and a string tied to the spoon of a grenade head on the back end is what sets it off. In the grenade head, where the primer once was, is drilled through and a nail or pin is placed through, protruding a slight distance out the bottom of the fuse assembly. The bottom of the fuse is screwed into the midlle of the closed end of the cylinder, and the 40mm round is then inserted in through the open end end of the cylinder, primer side down. The spring loaded striker on the grenade head is then pulled back, and held in place with the spoon. then a string tied around the cylinder and the spoon to keep it from going off. But then another string is tied to this hold down string, and stretched across an area when the trap is set up in the field. When one clumsily hits the strung up wire, it tugs the hold down string off of the spoon and releases the spring loaded striker. But, where there striker falls was once a primer, is now the head of a nail, on which the other end is resting against the back of the 40mm round. So when the sriker slams down on the head of the nail, it drives it into the back of the grenade, and sets off the valve.
So as long as you have the business end of the devise pointed along the trip wire, you can be sure that anyone who hapens to trips over it is going to have a bad day. Usually the 165 rounders work the best, because they have a much wider, and fuller shot than a lower capacity round.