score:6
Well...this is difficult to answer, because how many is 'common'? However, I've stayed in hostels in Australia, New Zealand, all over Europe, Central and northern Asia, USA/Canada and South America, and I can't recall a time that I've seen them charge EXTRA for female dorms - EXCEPT when there were fewer beds.
Now, it's fairly common to charge more for smaller dorms - fewer snorers, people to steal your stuff or whatever. It's more common to sometimes put an excess on some rooms to make them 'special' and include breakfast with just those rooms.
I've also seen it where single guys weren't allowed to stay in the mixed dorm room - only couples, which I'd say was bordering on discrimination. This was in Philadelphia, US. But it was the same price as the all-male dorm room.
I think, as @hippietrail commented, in most western countries this practise would be considered gender discrimination - charging people extra based on their gender, although I think a good lawyer could argue it's charging for security. However, for whatever the reason, I can confirm that no, it's not common at all, and if I saw a woman being charged extra for this, I'd certainly be arguing discrimination.
However, I've not stayed in many hostels in SE Asia, Africa or the Middle East (although I've been to a few countries in these areas, for whatever reason I stayed in guesthouses or hotels), and perhaps in those cultures it may be different.
Upvote:8
Yes. It's obviously natural and logic way to do.
If you have women-only dorms, you risk being not able to take extra backpacker even when you technically have free places (if they happen to be in female dorm and the backpacker is male) so they actually costs you more. You don't want to have a lot of female-only dorms so you want to discourage girls from using them and encourage them to choose mixed dorm instead (because it's cheaper).
Many women don't want to sleep in the same room with the unfamiliar men (for any reason), and it's why the hostels have the female-only dorms anyway.
It's not a discrimination that single-sex rooms are more expensive, because women are allowed to choose mix room if they want. Only because some of them don't want, hostels provide single-sex rooms as the extra service. I haven't ever seen a male-only dorm in a hostel, but it's simply because (generally speaking, in european culture) men wouldn't be interested in paying more only to have the guarantee to have guys only in the room (and probably more mess and more snoring).
Please note that my remark is for, generally speaking, 'Western-culture' countries. There are some countries which object unfamiliar men and women in the same dorm for religious grounds.