A strange size, 64oz. I presume that it is US fl oz like the Sweetman's growlers, so 1.89litres. If a pint is 568ml, a refill should be between:
(50% to 60%) X (Price of pint)X(1892.71/568.26)
So 1.67 to 2 times the price of a pint. €5.50 for a pint of Full Sail I believe, therefore €9.19 - €11 for a growler fill, €1.50 less with vouchers. Not too bad. Is Sweetman's still a flat €12 (€10 for Beoir members)?