I went through this with good results on my attic area in a catherdral ceiling. You can check out the post, info and pictures in the link below
MY ATTIC SPRAY FOAM THREAD
I do not have hard heating cost for you but we dropped about $800 a year in heating costs (propane forced air) and inproved our air leak test numbers by almost 20%. In addition rainfall on the metal roof is much quieter.
I am now looking to move ahead with the next stage which will include more spray foam.
Much like my attic space my walls were not vapor barrier well and have many leaks. In addition, due to the previous leaking roof, I am certain I have wet insulation in many places. Finally my house was built with 2X4 framing and was balloon framed (at least thats what its called around here). This means you stand up the walls on the basement block wall and then build the floor inside it.
Unfortunately when the builder did this, he did not install vapor barrier on the wall and then build the floor. This means the section of wall that goes below my floor level has no vapor barrier.
The other issue is that fact that we will be putting additions out both sides of the house in the next 5-8 years depending on funding.
So we will be spray foaming the front and back of the house with 3" of closed cell foam. On the sides we will be spray foaming the bottom foot to deal with the balloon framing issue there and using batts (new ones) on the rest, taping up the vapor barrier issues prior. I am then considering putting up 1" of foam board on the outside sides of the house to eliminate thermal bridging there and help out our heating bills until the addition is done.
Any thoughts here?