This is actually a great question. I suspect that the truth is: You can write off some % of the entry fee for the races that specifically ID themselves as contributing some portion of the proceeds to a charity--i.e., the Susan Komen races, or "Run for Pancreatic Cancer" or "Run for XXXXX Hospital."
Remember: not every race is a charity race, and even with charity races that have inflated prices, some portion of the entry fee is surely going towards "normal," non-charity expenses that are part of putting on a road race.
When I gave $125 to WBGO-FM last year as part of a fund drive, a significant portion of that, but NOT all of it, was tax-deductable. They sent me a letter to that effect, acknowledging the portion that was tax-deductable.
If you run a lot of races, and if a significant number of them are charity races with inflated entry fees (i.e., $30 for a 5K), I would keep duplicate entry blanks, and I would probably claim 60-75% of the entry fee as a charitable contribution. It's a lot of work, but if you're running, say, 30 charity races a year at $30 a pop, and if you claim $20 on each race, that's $600 in the "charitable contribution" column.
So no: you're not crazy for asking the question. In fact, you should contact somebody at the Susan R. Komen foundation and ask this question. Is some % of the race fee given directly to the foundation, as opposed to covering actual race expenses?