get CE (cheat engine(i used 7,2))
load up the game and CE step 1,2 and 3
choose the car u want, then look at the price for it (i.e 30000)
change "value type" to 4 bytes, if it doesnt work try the value "all" instead... step 4
type in the price for the car (here we use the 30000). step 5
hit "first scan" .. step 6
you will now get a bunch of result in the box marked 7. (u should only get like 20-40)
go to the bottom, pick one (u can double click it OR) highlight it, then press the small arrow (step 8)
now double clik the "value" part of the line (step 9)
a pop up will appear, here type in 0 as in ZERO. (step 10)
now hit "ok" (step 11)
go back to your game page, change the choosed car (doesnt matter if u use left or right arrow), then go back to the car u actually wanted again, AND it should be set to 0... (IF NOT)
follow step 9-11 to change the value back to its original.(u dont need to do this, but it can freeze the game, if u hit the wrong line)
then follow step 7-11 to pick a new line from the box
rinse and repeat,, eventually, u will find that your car is set to 0.. hit that ENTER and your golden

i know this is fairly long and seems boring or too hard - im a noob at guides and i have no vid recording programs.
also, ive already done it, so cant show it on vid. anyway..
if it by any chance DOESNT work, feel free to hit me up, ill be happy to help :)