Only including cars I've had on the road or intend to, not just bought for parts!
1st ever car in 1991 was a 1977 343 DL (parents old car) free!!! eventually stolen while S.O.R.N.
2nd car in 2001, 340 GL 120,000 miles, £500 high miles but good condition, now my everyday car
3rd car in 2002, 340,?? miles £30 good condition, given away because I couldn't sell it!
4rd car in 2009, 360 GLS 17,900 miles £1200 nice and shiny

sunny days only
5th car in 2009, 1980 343 DL 66,000 miles £150 project, needs a lot of work
I would say values depend on condition of course but also how desperate you are to sell or buy a car and how many people are looking for a car at any one time. With modern cars there are always hundreds or even thousands of people looking for a model of car. Hence we sold our scenic for £1100 within days, even though it was nearly 10 years old and just below 100,000 miles. A 50,000 mile 20 year old 300 would take longer to sell and not sell for as much (although it will probably hold it's value better if you look after it, unlike the Scenic, which being French, is rapidly heading towards the £250 bargain basement dept., hence the sale

)
How to increase the value of your 300:
Buy up as many 300's as possible and scrap them (wasn't that something thats come up on the forum recently regarding a certain TV motoring correspondant and the Ford motor company?

)
When you reach the point where more people want cars than there are cars.... £££
I think we are coming to the end of the age of frequent 'barn' finds of low mileage cars, but there are still quite a few others out there still in everyday use, even not including us!