An almost no heat pump usage month is $70, January and February might be around $170. Average is probably $120 or so.If it is going to cost you $6k plus to upgrade, how long is it going to take you to recoup that on your electric bill?
So heat pump is probably $0-90 a month depending on use. I don't know how many times more efficient a new pump is(I'm sure mine has lost efficiency from new, too) or how much rates will climb over time, but my guess is much more than the lifetime of a new unit.