Answer:
For Matchbox cars in the united states - PRE-SUPERFAST (ie. 1953 - 1968/9) here is how to arrive at an answer:
Although extremely rare versions (with certain wheels, certain axles, different decals, etc.) can easily range into the $200 - $300 range in mint condition, here is a rough guide:
IF you have a car in MINT condition with its original box in good condition:
1. 1953 - 1956 - $100.00
2. 1956 - 1959 - $80.00
3. 1960 - 1964 - $60.00
4. 1965 - 1968/9 - $45.00
For models which came in silvery-grey plastic wheels, grey plastic wheels and black plastic wheels, there is a premium of about $5.00 for the grey or silvery-grey plastic wheels.
People will WANT to sell you such mint condition cars for much more money - but IF you have mint condition cars you can certainly get (net) the amount I have shown above with no problem.
If you have a mint car/truck, but no box, knock off about 15% of the above.
If you have no box and the car / truck is lightly marred, knock off another 15%.