Hi @DannyB,
…probably more than you realise. Obviously cost depends on 1) how much energy each defrost consumes (including that needed to re-warm the circulating fluid and also the house, which loses heat during the defrost), and 2) how often defrosts occur. It also depends on how you run your heat pump - if you run it continuously you might get more defrosts per day.
Some manufacturers (e.g. Samsung) have put a lot of effort in to minimising both (e.g. including an economiser circuit in their HTQ range which makes defrosts quite rare, and ensuring very short defrosts with a sensible controller algorithm). I only saw a handful on mine in the whole of the winter, each lasting less than 5 minutes. I put a couple of relevant numbers in Samsung ASHPs: Anatomy of a Defrost.
Owners of some other makes curse defrosts for their frequency and duration.
Not easy, unless you buy a GSHP (or move to the Bahamas…)