Dd is almost 13 and she doesn't get a regular fixed amount in pocket money and instead will ask if there's something she wants, to be honest she very rarely asks for anything as she knows her Xboxlive, crunchyroll and spotify subscriptions cost me over £20 a month.
She loves gaming and anime/manga so I'll usually buy her a manga a month and dh will buy her maybe three or four games a year. I'll give her a tenner if she's going into town and maybe a bit more if it's a special shopping trip.. two months ago we went to an anime convention and I gave her £60 as it was an extra special one off thing. She saves her xmas and bday money, is well,aware of the value of money and offers to cancel her subs when dh goes through dry spells with work.
Pitching in around the house doesn't earn her any money as it's something she's expected to do regardless of our incomes (a friend did this with her 7 year old in an attempt to get him to tidying his crap away, he soon opted out of chores and thought giving up £3 a week not to do them a bargain). I also don't link it to achievement but do buy her something at the end of each school year, nit for getting good results, but usually something to relax over summer with after working hard, a few books, a DVD , a board game type stuff.
She can spend any money given to her on what she chooses, I might try and change her mind or encourage her to look elsewhere score committing if I think she's going to pay a higher price.
She has bought things that I'd personally think is a waste of money but played with for hours and hours, I think the manga she buys is a waste, but she loves them, reads them several times, lives organising them, they are worth every penny to her and she gets value from them.
She says most of her friends don't get pocket money and just ask their Mam and Dads if they want anything.