We too have been into the skateboarding thing for a few years now. My ds is 15 and all he does is skateboard!!!
I would start her on something cheap. Wal-mart, TRU, Target all have them cheap. If she does like it and wants to continue doing it - I would then move up to something more expensive.
Some of the online skateboard shops are running sales now on completes (as they call them) - where they are already built. You can buy the components separately, but then you have to put it together yourself - unless you are at the store buying it. Some of thoses sights are...
ActionVillage.com
CCS.com
skateparkoftampa.com
galacticg.com
I try to get them as cheap as I can - I know some are selling the brand name completes for about $89 right now.
There are usually a skate shop in every town nowadays. We have several, but beware they usually charge more - from my experience. If you happen to live in Orlando FL the GalacticG shop I mentioned above is an actual store and has really great prices. We stopped in there this past August while we were at WDW and went ahead and got my ds school shoes as their prices were so good!
Good luck! Skateboarding can be expensive!!!!