This method is not one I found myself, however the lack of there being any actual guide with tips on how to make money quickly for the achievement made it significantly harder to find out about this method. The purpose here is simply to make the information more obvious.

Credit for the person who I found this method out from to Dikra on the discussions board.

• Buy the base coat tool.
  
• Buy one of the base coat options for the T-Rex. The first one is usually best as it saves a couple of seconds on choosing the manual.
  
• Buy at least one T-Rex model kit. It will be significantly faster if you buy multiple kits to do back to back instead of creating and selling one at a time.
  
• Make sure you have no other unmade model kits other than T-Rexes. These will simply serve to slow you down when selecting your model kit to make, adding extra time for each.

Now for what you actually do

• Start building the T-Rex model.
  
• IGNORE THE MANUAL FOR NOW.
  
• Assemble the model using the quick assemble [+] in the parts list.
  
• Use the base coat to paint the T-Rex.
  
• START FOLLOWING THE MANUAL FROM HERE.
  
• Apply the decals and do the washes and highlights. You do not have to be accurate or neat, just make the manual page go fully green. You do not need to rotate the model during this stage, so any rotations to apply details are wasted time.
  
• As soon as there are no manual pages left, finish project.
  
• Sell the T-Rex at the auction house with a long auction. This should result in a 7 minute sale usually selling for between $850 and $1100.
  
• Rinse and repeat until the achievement unlocks.

If you follow this method, you should be able to be completing one T-Rex approximately every 2-3 minutes, maybe even faster. With the $15000 you should have gotten from completing everything in the campaign and the other achievements, it should as such take around an hour to an hour and a half to get to $50000 earned. Now you can sit back and enjoy your ribbon.