It very well could have went up $20/month. It depends on how many hours/day you have the truck plugged in and your cost per KWh.
You need to take your cost/KWh (Here in MI it's $. 082, and multiply this number by . 72 (The heater element on our trucks consumes 720 watts).
For me, 2 hours/day, 30 days in a month, is about $3. 50 per month. Now I don't notice the $3. 50/month, but the colder it gets, the longer I leave it plugged in, and the higher the electric bill gets.
-Ryan