price will varry on size of canvas, cost of colours, labour and if you have ever sold any or had any exhibitions etc. if she is a novice and only been doing it for herself till now then she could sell for $50 - 150 each depending on how much it cost to paint. you can advertise your stuff on
www.deviantart.com or just exhibit it buy signing up for free and seeing if you get any interest. its a very good site to get your name out there. you can start a market store up and advertise and see if you get much interest. maybe small paintings at $25 and larger paintings for $80 for the markets till you get your name out there then start to up your prices.
just remember as long as your making monie and not loosing monie then thats the goal, if you sell for too low then you might not sell much to keeping your price at least double or tripple the amount you spent to put it together. people at the markets will have less to spend on them so will pay what they have or you can take orders and get a pepossit down and just call them when its complete to pay the rest.
I've done a few markets and have had a few exhibitions and so forth and the max I sold a medium sized painting for was $55 and large $80.
I sold alot of smaller ones for $15 - 25 each depending on the amount of time I had put into them.
good luck. maybe go to the markets and check out other peoples prices and ask questions such as how long they've been doing it, and if they've got any exhibitions going etc. if they just do it and haven't had any exhibitions then go by there prices but also look at the detail of there work and compair it with your girlfriends style.
its a very nice painting what was it painted with looks to be pastel? pastels can sell for a lil more then an acrillic painting cause there's more work into them and its alot harder to paint with.
the most I've payed for a painting was $180 but it was from a older woman that had had many exhibitions and been painting for 40 odd years and that was classes as a small painting.
also take into consideration a framed painting is worth more then a canvased or non framed painting, its also looks much nicer when fraimed but a canvase painting can also look fantastic on a wall as well. if you are using canvased then adding price of canvas, paints and time to it will varry on the cost you sell it for.
hope this helps
cheers and good luck