Currently the race rewards for 2nd and 3rd place include 3 million food. If you were to go into the shop and buy the best food offering that would be "Catering" which gives 140,000 food for 150 gems. So the race gives 21.4x as much food as "Catering". (3,000,000/140,000=21.43) This means the gem cost for that much food is 21.43x150=3,214.5. That's a lot of gems.
If in addition, you are getting personal rewards and you have maxed out at an additional 5 million food. That food would cost (5,000,000/140,000)x150=5,357.
The grand total would be 8571.5 gems. It would seem to me that if you think can afford it, it is quite reasonable to spend gems to get food.