If the horn is genuinely damaged there are safety and efficacy issues, but if not usually the most you need is a washer unless you are choosing to upgrade to a Frost Free horn (but then again if you are adequately training staff they shouldn't hold the horn).
There are some interesting prices paid by people out there- a certain fishy company is charging £85 to refill a 6 litre foam extinguisher - cheaper than their new ones at £150+ but you could get two or three new ones from other suppliers at that recharge rate.
It's not new for prices to vary widely - recent additions to the museum collection show in the early 60's a 5lb CO2 extinguisher was around the £9 mark, yet from a certain west riding firm of note it would be £19,19s!