It's cheaper for "next day" than for "unlimited". As Inverse says, it was £8 last year for a following day ticket.
You're supposed to have a ticket from entry the first day to get the next day one, but we often buy them if we're at the park for people coming with us the next day. The people in the booths selling are pretty easy going - just say you lost your entry ticket.
Even bought the £13 unlimited ones for people not wanting to faff with bogoffs.
There have been price rises though this year, so will be more. I'll try and remember to find out the prices while I'm there on Saturday.