The entry fee's of tournaments vary, some are diamonds - some are coins. The largely depend on what you are racing for.
We create tournaments for coins and diamonds to give players who do not have the means or the time to race - earn - save and spend to upgrade their bike, these tournaments are a great way to accelerate your way through the game.
Some tournaments are for real world prizes often costing over $600 - for these sorts of tournaments you will see the entry price is mostly in diamonds
Every time you race in a tournament you spend the currency listed on the entry fee button. If it is a 5 diamonds entry fee - every race you do costs you 5 diamonds.
To be clear - every time you race in a tournament you are charged an entry fee.