I Have made a small tutorial for editing your TDU savegame so you can add about 1.000.000.000 $
1. download a hex editor program like Hex Workshop
http://www.hexworkshop.com/
2. back-up your save (in case something goes wrong)
3. open Hex Workshop (or your hex editor) , open file playersave from My Documents\Test Drive Unlimited\savegame\username\playersave\
4. overwrite hex addreses 00000038 to 0000003F with FF
(Simply goto find and choose hex, then type 00000038, replace the two charcters with FF)
5. save it
6. open Hex Editor(or your hex editor) , open file playersave from My Documents\Test Drive Unlimited\savegame\username\playersave2\
(note: now edit playersave from directory playersave2)
7. repeat step 4 overwrite hex addreses 00000038 to 0000003F with FF
8. save it
9. enjoy your millions of dollars :P
have fun! P.S. it tooked me 2 days of evaluating values from 20 different saves :|