We priced the tokens at 0.08 ETH (not including bonuses) to give early advertisers a large incentive to use/burn their tokens for the discounts (since face value is 0.5 ETH). This helps us build the network more quickly, making the product better for everyone. Logically, if the network is successful, then the tokens should be bought by advertisers at around face value.

The face value of the token/coupon is arbitrary, and is related to the token supply.

Because the face value of the coupon is denominated in Ether, the tokens are worth more to advertisers if the price of Ether goes up.

Thanks for asking a good question.