I'm no expert but I've been researching this as well for our new home. I've priced out a bunch...1/2lb open cell, 2lb closed cell (like Mike Holmes always uses), as well as the envirofoam (soy).
The 1/2lb is by far the cheapest but then adding in venting and vapor barriers plus labor it gets pretty close. To give you an idea our Cathedral roof is about 1800 sq feet and we have been quoted $7000 for the 1/2lb, and around $10 000 for the other two. The more you do, the cheaper it gets so we've added all joist ends, cantilevers, and basement walls for a total of around $14000. This is to an R40 equivelent everywhere except the basement R22.
When getting quotes be sure to have them specify the thickness, some will give you a quote per sq foot per inch, where our roof is to be 6 inches thick. I have some great links you might want to check out:
I believe the soy is a good product but as far as being enviromentally friendly, the actual soy content is quite low and the reason rain forests are being clearcut for soy farming...but that's another topic