sounds like they're trying to charge you about 2-2.5 hours of labor. it's something you can do yourself. you drill out the lock tumblers with a small drill bit/power drill.
to keep the keys matching you'd buy a 3-up keyset, it'd be for the column lock, glovebox, and seat if you'd like it.
http://www.scootermercato.com/Scooter-Parts/Locks/179292
http://www.scootermercato.com/Scooter-Parts/Locks/178558
i posted american links as im not familiar with uk/euro px links. more than likely you'd remove that cover, drill out the lock, replace the lock, and put the new cover on.
to be honest ive never had to do it on a px, but i've done it on a pe using this tutorial, some of which will be suitable for you:
http://www.scooterhelp.com/tips/body/locks.html
there are a few forum topics about this and maybe a blog online, so just look around for a proper step by step:
Ignition Barrel Replacement **HELP**
the ignition switch itself is retained by a single screw but you'd have to drill out the glovebox lock and remove the horncast with a screwdriver in order to access it. again, i've never done it so i'm not sure if you'd need to remove the ignition switch when drilling just to be safe:
http://i.ebayimg.com/t/Vespa-PX-Ignition-Switch-Key-LML-Stella-125-150-200-neck-lock-Plug-/00/s/NzUwWDEwMDA=/$(KGrHqN,!k0E68SV(UMEBO9z0BKpmg~~60_35.JPG
heres a glovebox lock replacement tutorial thats universal:
http://www.scooterhelp.com/tips/body/glove.locks.html