You pay whatever the increase is from what you actually paid for your tickets (If you got a discount of any sort it shows up when Disney reads the bar code on the ticket) Here are the regular prices, but I bet you paid about $20 less to start out with:
3-day Park Hopper - $159 for adults; $129 for children (ages 3-9).
4-day Park Hopper - $189 for adults; $159 for children (ages 3-9).
5-day Park Hopper - $209 for adults; and $179 for children (ages 3-9).
Do this at Guest services, last time I needed them they were in the Bank to the right when you enter Main Street USA. There can be a long line sometimes, so if it is short jump in. If your ticket has an early entry feature I would use it before I upgraded, I don't think the gate tickets have that feature so you would lose it otherwise.