we pay €350 per year rental for the machine. Also about €100 for ribbons.
I find it's main advantage is convenience.... not having to have stocks of stamps on hand. We also send varying size packages which can be very awkward using stamps.
It can be topped up by bank transfer and an automated telephone code.
An post give a nominal discount of 1c on a standard letter which is franked.
Our postage spend would be around €400 per month so it's a necessity for me.