Subletting fees for a 4 person house?

I rent out a four person house with 3 of my friends, and this summer we are subletting it. We each pay about $450 per month, and a young couple want to sublet it. Our landlord referred them to us because they are friends. She suggested we rent it out to them for around $700 a month, which wouldn't even cover two people's rent. Obviously it's better to sublet it and make some extra cash if we're not using the place, but does this seem appropriate? Should we be asking for more money?