data,
yup the 135's are a bit of a rip, even 2nd hand it's cheaper to go for 2 250's rather than a pair of 135's. however the 135's were phenominally popular amps so the market could obviously bare the price and i suspect in part the money gained pays for things like keeping stocks of obsolete components to service older kit, r&d for the reference / 5 / 500 series components, keeping the lower end kit at a 'reasonable' price. also the fact that naim is still around whilst so many of their contemporaries are either no longer about or have been bought out and are now made in asia. also you seem to have forgotten to include perhaps the most expensive component in your assessment of thier cost - labour, you're paying for twice the labour which in the uk isn't cheap either - although admittedly not enough to warrant such a large cost discrepancy.
cheers
julian