jcsw - I had a look at the Warbeast template and it was waaaay too cheap for the creatures they're buying. 11,400 for 18HD? Umm, yeah....... Not happening.

Another-Poet - The thing about the spell method is that there is a limited list of creatures on thosr summon lists, and the CR of a creature is only an approximation. Any creatures not on that list, but of an appropritate CR may be worth more or less than that magic item would dictate. I've seen CR 20 magical beasts taken down by a level 15 party, and level 18+ parties have trouble CR 15- magical beasts. Because of that massive variation between creatures, I'd shy away from the magic item method. It is a simple and easy way of doing it though (At lest to me).

Doug Lampert - Interesting way of doing it, but I'm not sure how you came to that method... You lost me about halfway. I see how it works mathematically though.