It's quite easy to find a used car ending in whatever digit you have, then try and work the last 3 digits to 750 or 500, the rest will be easy then. I got the Penniless from about 460,000 (most on a Murcielago), think it through and it's not an issue, though the earlier you do it, the easier.
Last night I read about this Trophy, so I decided to go for it as well. My method was similar to Sureboss'
I have already raced up to level 15, so my credit balance was about $472,006 (I don't remember the exact amount).
I looked in the used car lot for cars that would reduce the odd $6 amount and saw a Nissan Skyline GTR V-Spec N1 R34 '99 ($37k) and a Mazda Atenza Touring Car ($94k) that had the required digits so I purchased both. After these purchases, I had $340,160 exactly.
So knowing that I would shortly need a Ferrari (don't we all?) for some Italian races, I went to the Ferrari dealer and picked up a Ferrari 430 Scuderia '07 (for $302,500). This got my credits down to $37,660.
I then took a look at the Subaru dealership and found a beautiful Subaru Impreza WRX STI '10 for $37,380, so I handed over my cash. This got me down to $280. Almost there!
I didn't know of anyway to spend $280, so I did some quick calculations and decided to earn a bit more cash.
I hopped into my Daihatsu OFC-1 Concept '07 and ran two races at the Beginner Lightweight K Cup Series, and twice received the $4,860 prize winnings. This increased my credits to an even $10,000.
Next up was a trip to the Tuning Parts store, where I picked up some Dirt tires for $3,000 and some Sport tires for $7,000.
My credit balance went to zero, and I'm Penniless!!
The Trophy is mine!
Enjoy the racing,
GTsail