That is to say, the value of a second roll, how much of a bonus value is it worth.
For example: when you make a saving throw, attack or check roll, when does +bonus to the roll and rolling twice equal the same worth.
I believe the value of a second roll would be different depending on if it was 'Choose the Highest', 'Choose the Lowest', 'Choose any Result' and 'Reroll on Failed'. Though in this case I'm thinking for 'Choose the Highest'.
Edit in response to comments; For the intentions of this question the dice is a d20.