Hello Victoria, I cannot tell you exactly how much it costs /sf to install foam but I can tell you confidently that you will see significant savings in your electric bill. If your are considering this for your attic, I suggest blown in insulation since it us a better option for the nooks and cranny's of a basement. Along with this if you also check and fix drafty windows it will help bring down the cost. These homeimprovements cost upfront but is a worthwhile decision.