Being that I work for one of the world's largest IT providers, your suggestion to ask other industries about their average IT spend is a bit unfair. Specifically, most industries look at IT spend as a percentage of revenue. Being that car dealers sell one of they most expensive retail items available (with the most probably being real estate), you have to be very careful in making this comparison.
The good news at the end of the day is that with the advent of ASP solutions and the move to common off the shelf applications, this industry segment will see reductions in IT costs for the average dealer. Every model we run with these two factors in place bears this out. It just makes sense that by consolidating users onto larger systems and reducing licensing costs, sys admin costs, data storage fees and others (combined with very inexpensive communication options) that dealers will see an advantage as this new model plays out.
As a last response to Kevin's comments about the consolidators not writing their own DMS. I believe most would albeit for one sticky issue - factory communicaitons. This is the artificial barrier in the industry that keeps new solutions from being viable. Without 'DCS' functionality (integrtation), new offerings will continue to be thought of and possible built, but are doomed to failure.