I have a professional trainer come out to my practices once a week. She runs a 45m core/agility program after our practices. It comes out to about $5 per kid.
I've found that having an outside certified instructor doing this helps a great deal. It allows me to feel less like a PE coach "inspiring" (read motivating/yelling/pushing) the girls through the workout and it gives my staff time after practice to observe, learn, and discuss. The girls seem to enjoy having the trainer, even though some think she's "mean". She brings her own agility ladders, ropes, hurdles, ect, so at the end of the day it really isn't more expensive then investing in agility gear for the team.
Barring that, the Nike Sparq series has some softball/baseball specific core training. I learned a good deal from one of their videos and still incorporate some of that into practices today. You can make a good conditioning program from that. You can probably find a SPARQ agility equipment set at play it again sports.
Its a dvd but the dvd just demonstrates the exercises, you follow a chart for the actual workout. I bought it for my kids and its a great workout designed to build an athletes core.