I think it depends on the value of the car that you want to race and the level at which you expect to compete. When I decided to take the leap and go to comp. school people told me it would cost $1200-$1500 to rent a car for the school. Since I was a beginner, I knew I would be content in the back of the pack for a while and the only important thing for a school is that the car passes tech.

So I went out and bought a back of the pack car for $5000. Two schools and two races and I will have recovered the cost of the car in what I would have paid in rental fees.

When you are renting something occasionally that the owner can rent out to others when you aren't renting (like specialty equipment at a rental place) renting can be cheaper. But there are only so many race weekends in any particular region, so it seems to me that the renter of a racecar is brunting the majority of the cost of operating that car. A cost that now includes the labor of the owner/crew and well as the profit for the operation.