To give a value to a specific brochure is very difficult. There is a misconception that the highest quality items have the greatest value. There are many factors that will affect values. These can include:

Brochure Condition
Manufacturer
Model
Print Run or Availability
Brochure Quality
Production Costs
Current Economic Climate
Current Market Demand

As you can see, if you take all these into account, getting an accurate value is tricky. There is also the human element. One persons idea of a value may be different to another, even if they are taking into account all the above factors.

For example. An Aston Martin Vanquish S VIP brochure was produced around 2006 which had an opening hardbacked folder with 3 individual brochures inside, its own packing box and the overall quality was very high. In 2008, the average selling price on Ebay was £25. The production costs alone would have been £10 each, not many were produced, Aston Martin is a prestige brand and the market was fairly bouyant.
Another example is a UK brochure for the VW Golf Rallye. Produced around 1989, these brochures are simple A4 sized, softbound, around 20 pages and standard quality. And yet they currently sell for over £30.

This is proof that sometimes, a very ordinary looking brochure could have a greater value than one of a higher quality. As with Antiques, the value of car brochures will be driven by Market Demand. When the supply of certain items dries up, this will drive up the price that collectors are willing to pay. There is no printed price guide like Millers Antiques Guide, so you do have to be careful. Items can be under or over priced, so research is very important.

Car brochures will always have a value. Over time, a great number will increase making them good investments for the future. If the car they represent gains cult status, so will the literature.