Possible Clues:

• 50-50 chance
• 50/50 chance
• A 50/50 chance ... or a description of the lengths of this puzzle's Across and Down answers, respectively
• Toss-up at the track
• Paradoxically named bet
• Oxymoron at the track
• 1 to 1
• They're 50-50
• Ratio for 25-Across
• Toss-up ratio