Shops will eventually charge less for payments done in Bitcoin as opposed to dollars because of these three reasons:

1) Bitcoin payments from the customer can be immediately received as USD payments to the vendor (using a service like bit-pay.com)
2) Bitcoin removes the credit card fees of 2-3%
3) Bitcoin eliminates the risk of chargeback, which for many online vendors is a significant cost.

Combine these three factors and you will see Bitcoin prices will become lower than USD prices in the near future. This phenomenon is what will drive Bitcoin to widespread adoption, because web shoppers will start seeing two prices, one always slightly less than the other.