Well, speaking for myself alone, but still attempting to represent the non-uber-save runners...
Since I run on my own game account and not a super-save, I find that my limit to spending is based purely on the value of the car after the week is over.
For example, if you ask me to buy a car I already have in fully modified form and in the week's race you want it at stock weight, I would probably avoid the race.
Spending all that cash over again so I could use a stock car instead of my weight reduced version would be sorta annoying.
Also, for cars I might not have (although I can't think of any since I have all of the good street cars in GT mode) it would be a matter of whether I wanted that car as an addition to my garage.
If you didn't have the DB9 in GT mode and you asked me to buy it for the week, even with the hefty price, I would do it because the car is worth having in the home garage.
However, if you asked me to spend 100,000 on a '98 SL500...
I would probably save my money since the car isn't really that great (especially compared to its slightly more expensive new generation counter-part).
Now, the kicker in all of this is the price itself.
If you asked us to buy a car for 40,000 I would have very little trouble doing so because the money for fully tuning it and buying it would be easy to win in just a few races.
My bottom line:
On a car I didn't like the max I would spend is about 100,000Cr (after tuning, not including tires).
On a car I like there is just about no limit to what I would spend.
Oh and btw, I don't have any racecars in my home garage so that's probably a major problem in this for me.
Coming up with the money for a street car is usually easy, getting the money for a racecar.
What, the cheapest racecar is probably 300,000 or something?
But that reminds me of a suggestions box post I've gotta make.
Anyway, hope that gives you an idea of where I'm coming from (as well as where many of us "original save" gamers come from).