You'll lose more than half the time. If you try a more complicated betting structure than "Bet you $5", you'll have to explain it to them, which will make them suspicious.
"I'll bet that if you pick any random number from 1 to 20, I can guess it. If I miss, I give you a dollar. If I get it right, you give me ten."
Over 100 trials, you would lose $1 92 times, but gain $10 18 times: net gain of $88. It works almost all the way down to getting $5 per correct guess.
Intuition would tell the guesser that they've got a nineteen-to-one chance of gaining a dollar. You know (in theory) they've got only slightly better than a one-in-five chance of winning.