A bullpen complete with all the fix'ns can be a fairly expensive deal. On the other hand if you looking for the basics, any raised surface will do, with a pitcher's rubber firmly attached to a poured cement block.
However, before I go any futher with some details, you should consider the following:
1. What's the frequency of use and who's going to have access to it.
2. Will everyone share in its maintenance and up keep, or will that be
the responsibility of paid employees of the school system, etc.
3. Will this area be open to vandalism? Is your school/location susceptible
to "get even" kind of activity after football games, etc.?
4. Are you and the other users of this place willing to conduct fund($)
raising to pay for the majority if not all of this location?
Ok, now for sticker shock-------------
A fully appointed bullpen with two mounds, screened in area, a home plate, bench(s), insurance coverage*, will probably set you back about seven thousand bucks ($7,000).
However, with a little --- ahhhh, we really don't need this - n -that, you could squeak by with about three hundred ($300) for a decent bullpen area. And if your really short on the ole-doe-rey-me, you could put together a place for about sixty ($60) bucks.
Why such a wide variation in cash? Well, a fully outfited bullpen area is just that -- A Bullpen. It's marked off about fifteen(15) wide and seventy(70) feet long. This space accomodates two pitchers with enough room for a pitching coach(s) to place himself/herself behind the pitcher or catcher, it also allows enough room between two catchers to work without distractions from his/her counterpart on the left or right (catcher). The two mounds are each abbreviated surfaces just large enough to accomodate a rubber, the plateau, and a slope. The bill of material for the mound is usually mound mix/crushed brick/Bean clay, hardeners, and moisture absorbents. The final touches are addressed with a cover for each mound.
The home plate area is basically made up of the same material only flat and pounded very hard giving the catcher a firm platform to "set in".
All of this is caged in, a bench is provided with a roof for shade, equipment holders for baseballs/clipboards/equipment bags/ rosin socks/cleat picks/ water coolers, etc. And if you really want to get fancy - go to Radio Shack and pick up a couple of cheap walkie-talkies and be just like the Big leagues!
On the other end of the spectrum --- just go to your local sporting goods store, buy a pitcher's rubber for under twenty bucks ($20), purchase a bag of cement and some boards and make a form about the size of the rubber, screw the spikes onto the rubber, pour the cement into the form, wait till its workable - not watery, push the rubber into the cement form, go to your field and dig a hole so the rubber is no more than five inches out of the ground, get some fine crushed brick and mix it with bean clay and dirt, make yourself a gentle slope, at about sixty feet away place a piece of plywood cut to resemble home plate. -- and wallla!
Instant bullpen. Don't forget to cover your pitching surface so rain won't destroy your hard work and $$.
Depending on the insurance policy(s) in effect on the property that your considering to put this bullpen on, you may be restricted on building a thing like this when all you have is a makeshift moud that is off the field of play- proper. Public use of your field in any way where such a area is open to any kind of public foot traffic may cause you some problems. Maybe? So, you my be required to fence this area in so as to restrict access. I hope some of this helps. Great post I might add - excellent thinking.