The banner is more effective the better the save of the unit it is given to. More than with most other items it gets better as the unit's stats get better. Maybe make a houserule with your opponents that the points cost of the banner is: 25, 40, 50, 65 points, depending on whether it is given to a unit with a -, 6+, 5+, 4+ save (respectively)?
(adjust points as playtesting indicates, of course. Maybe something like 25, 35, 50, 70 would be better?)