We come down very hard on lying - grounding, no friends round etc for specified times. DH also does the loooooong version of an incredibly boring talk about how we're preparing her for adulthood guaranteed to upset her though he doesn't raise his voice at all.
The reasons we come down hard is because we feel we can't keep her safe if we can't trust her to tell us where she is; we also feel that she needs to learn to negotiate; and feel it's disrespectful as we don't lie to her.
I would start by disbelieving all of the above and not giving her room to lie to you, just assume she is. Start with the pizza.
"You said it was £4.99, it's £9.99 - you're now grounded for a week"
I also give her chances to come clean about stuff and time in her room to think about coming clean and I tell her the consequence will be less if she tells me the truth now. I tell her that she doesn't know if I know yet so think very hard (hard stare) about what she is going to come clean about.
I don't argue either once I believe that a lie has been told, there is no wriggle room and she doesn't get the opportunity to convince me - when dd was younger she swore til tears and blue in the face that the creme egg gift set had only come with one egg and that whoever packed it was clearly rubbish at their job.
I knew without a shadow of a doubt that she was lying - grounded her for Easter weekend and sent her to her room immediately as she was hysterical - an hour later she comes down, says she was lying and she was sorry.
She was genuinely contrite and that's one of the things we look for.
She definitely lies less now. And now she is older I labour the disrespectful to me line, how would you like it if I lied to you etc and I give her examples that make her think.