Entry is 5-7k depending on track. Driver usually gets 25-40%. You also have to factor in cost to prep car, as well as sufficient parts, backup car etc. Even taking your figures for tires and fuel, which are low (Tires about 2k a set of four, need at least 10 sets at very minimum for weekend), you aren't making money on running last.
Unfortunately, the reason they dropped out of several races is because they couldn't even afford enough sets of tires for entire race, and also couldn't even afford full pit crew. Also, when minor parts break early, as they almost always do (i.e. clutch at Darlington), they either don't have the spares to replace during race or don't have manpower to change the parts. So, they retire the car.
The fact is 50k gross helps nothing. Sponsors are needed. Sponsors pay the above expenses. Besides, they don't have a car that can run a competitive speed at a Pocono type track. They would likely have to take the Charlotte car which was terrible, and which ran about 2.5 seconds slower than it needed to on a 30 second track. So, they were looking at being lapped every 15 laps over the course of a 600 mile race. That is not fun, is dangerous, and does nothing to help sponsorship (aka Shawna Robinson).