Roger,
Valuing a rug is such a dodgy area to get yourself involved in without the necessary credentials. I certainly wouldn't do it!
It depends how you look at "value"
1) Retail value new - how much you would pay if you walked in to a shop and bought the nearest replacement for it brand new
2) If no exact match, Cost of something of comparable appearance. properties, age, condition
3) Calculate what was paid for the rug minus depreciation over the years (these would almost certainly not appreciate in value in the same way as pre-world war 1 Persian rugs would)
4) Market Value - what money would realistically change hands if the rug were bought/sold on the open market at today's prices.
I'm afraid that you should probably decline from getting involved in this. If it needs valuing it should be left to someone with the necessary appraisal skills. You could end up shooting yourself in the foot in terms of your client's perception of you.
I have the utmost respect for you Roger but asking on THIS forum for people to spit out ideas of prices for something 99.9% of people know absolutely JACK about
Best bet might be to give Alan Harrison at Olney Rugs a call, see if he can help out. 01234 712502