On the basis that 28 people have viewed this topic and no-one has posted a reply, I think you've sort of answered your own query Rob.
There is certainly an element of secrecy when it comes to prices and some people might also think their car is worth more than the market currently dictates - it's very subjective.
The general topic of 131 prices and values has been the subject of many a thread on this site and many an hour has been spent in face-to-face discussion on the matter too. The simple truth is that values vary enormously between any given car and condition and originality is generally more important that mileage.
The global geographic location of a sale will also affect a value as will colour. If a car is painted in a non-factory colour it is less likely to sell quickly than one that is - as was recently proven with the slow sale of a blue 131 Sport, and even a colour change (say from silver to orange) may also affect the value.
Owners of any classic car are usually less inclined to publish the price they paid for their car as if/when they come to part with it potential purchasers may get wind of that original value and try to beat the new asking price down.
So, as regards values, who should do the valuation? The owner or an 'independant' valuer? The difficulty with the latter is that they generally have no idea what a Fiat 131 mirafiori is let alone what one is worth! However, at the risk of being shot down in flames
I think that generally the following is true -
1) Most people looking for a 131 are looking for a Sport/Racing and would pay more for originality.
2) Twin Cam 131s generally fetch more money than either OHV or single OHC engined cars.
3) Two door cars generally fetch greater values than four door cars.
I hope that this posting helps Rob.
Cheers
Simon