Given the last brand new Brum Tempest I bought would have been from JSR mail order in the mid-90's for around £75, the Bank of England inflation calculator has the equivalent new price today at, give or take, £141.
So technically £100 or so might not be unreasonable boxed S/H in good condition - but I agree they are undervalued at that. It seems the price for a fairly nice boxed example with paperwork currently retails in the shops for between £200 to (gulp) £275. Obviously private and fair sales are less, but ...