Entry fees are NOT contributions
Runners CANNOT deduct race entry fees as charitable expenses on their taxes because the payment is a registration, not a contribution.
Even if payment is made to a not-for-profit organization, it is not a contribution because it is mandatory and payers are receiving something in return. It is a purchase, not a gift.
Registrants can only deduct the amount of the entry fee that is greater than the cost of what is provided to them. Since entry fees rarely cover costs, usually nothing will be deductible.
Even if you raise enough sponsorship dollars to cover all costs, the costs have not been eliminated.
If, for example, a race with 500 registrants costs $20,000 to produce, the cost is $40 per runner, no matter how much money is raised from other sources. So in this instance, participants can only deduct the portion of the entry fee in excess of $40.
If your entry fee does exceed the cost per runner, IRS regulations require you to disclose that amount (just as you would for a fund-raising dinner) so participants will know what amount actually is tax-deductible.