hi, they can be kept in pairs not sure about groups, but males and females together will mate then fight sometimes to the death, two females are not a good idea either - males are usually more placid. You will need a suitable cage - ie plastic or bars closer together than cages for syrian hamsters.
I think they're about £7 each but could easily be wrong!
They often don't sell males as pets (for aesthetic reasons) and they recommend putting sisters together. However, I have heard of a lot of people geting 2 females but then having to separate them because of fighting. I just got one for my daughter and it is a lone female - I know lots of people are against this, but I don't want to have to deal with them trying to kill each other. Don't underestimate their speed - they are really really fast!
Personally I would stick with a Syrian.
I have kept both. I bought two "female" dwarf hamsters but ended up having to separate them as they were in fact boys and kept fighting. They also bit. That said I have looked after someone elses dwarf hamsters and they were adorable.
Still I think I prefer the chunky ones :)