The price for two PA28 inertia reel shoulder harnesses including fitting is unfortunately the going rate.
Pacific Scientific who manufacture the PA28 shoulder harnesses have a UK office in Maidenhead.
A few years back, you could get a legal overhaul of the units for around £150. Last time I used them the price was very similar to the cost of a new harness so no real advantage given that you have to wait to have your belts overhauled.