For our ChumpCar effort (a similar series to LeMons) we're trying to keep it to about $500 per driver and we have 6 drivers. That doesn't include the driver's personal safety equipment (suit, helmet, shoes, gloves, neck collar), just entry fees, car cost, parts, tires/wheels, and safety equipment. That is about $3000.
Since I didn't have a legal helmet (needed a closed face and my new AutoX helmet was not) or a suit, I had to buy them. Since it is near the end of the season, I was able to find some decent deals and bought an SA2000 helmet and a current suit and gloves for about $200. The shoes are another $50 and the collar and Nomex headsock will probably be another $50. That's $300 more per driver which potentially brings the total to $4800+ depending on the cost of your personal equipment.
BTW, looking at the LeMons rules you can share safety equipment (while gross) is legal, but in ChumpCar you can only share driver suits amongst drivers, each person must have their own helmet to ensure a proper fit. Plan on at least shoes since foot size is pretty variable.
Finally, this doesn't include the track support equipment, truck, trailer, tools, food, gas, spare parts, etc. The truck/trailer could be borrowed or rented and the tools/spares can be tricky if you have none. If you don't have these answered already, you're in trouble.
Good luck!