. . . the default speed is 100 units / s and the world grid is 100000 x 100000 units . . .
If I understand this properly, that works out to a total of 10,000,000,000 locations to hide the prize, and at the default speed a player can search 100 locations per round? Each player has a 0.000001% chance of finding the prize in each round. (1 chance in 100,000,000) If you have a single player, he has a 50% chance of finding the prize in 50,000,000 rounds (assuming he can avoid re-checking locations that have already been checked. At 0.1 BTC per round per player, that means the total prize will be 5,000,000 BTC after those 50,000,000 rounds.
It sounds like a game some people might play, but you should probably either decrease the cost per round, or decrease the size of the play area (or both). If the game becomes popular you better be sure that you can securely store 100,000 BTC or more for the prize. You will have a lot of very angry people if the prize pool is hacked and stolen.
To make sure the game is fair, you probably need to announce a hash before the game starts that proves the location of the prize. You'll also need to find a way to convince everyone that you aren't stealing the prize money yourself.