There isn't an answer to this one. It depends on how well your house was ventilated before. Keeping the vents shut and living in a moist and mouldy atmosphere is more cost effective than ventilation of any sort, but not great for your health.
So this is a quality of life issue rather than an economic choice. You will have a well ventilated, drier and possibly warmer atmosphere in your house, but it only saves you money if it is replacing a very effective ventilation which was causing draughts before, which were being warmed up by the central heating.
MHRV won't work very well at all unless your house is pretty airtight, and the work of improving your air tightness will also reduce your heating bill - or increase your comfort level.
But nobody asks the payback time for a nice fitted kitchen, or a new car - its all about comfort to my mind. Anyone selling MHRV on the basis of payback is well dodgy, unless they are willing to do a very comprehensive analysis of the ventilation and cooling of your house.
