Hi Sacha
Rose gold is no more expensive than other colours of gold. However, not everybody use/sells it. In the past 2 years or so, it's become much more popular, especially in the US.
Personally I'm not wildly keen because (a) I like to co-ordinate and don't like rose gold enough to buy matching items (b) for me, it's a bit "flesh" coloured and (c) I think it compliments some colours i.e. pink diamonds by bringing out the pink but it also clashes with the majority of other colours. Since I don't like things to clash - and I don't believe a setting should overpower a stone - I generally only use rose gold or accent diamonds OR where I simply haven't got a choice.
The are also different shades of rose gold depending on the alloy mix. It can look very brassy and orangey, it can look like a subtle warmer yellow gold or it can look quite pink.
I'm in the minority because I think most people like it but it's very much a personal thing.
Here are a couple of pics. First up is the Kunite ring from GemsTV with dire diamonds. The head is rose gold. Then there's the champagne diamond necklace. I love the necklace and thankfully you can't see the rose gold because it's smothered in diamonds - however you can see it on the reserve side. I will eventually have the rose gold rhodiumed away but for now it's there. Lastly there's my pink/white diamond pendant. You'll see the central pink diamond is bezelled in rose gold. I've got several rings that I've done the same with and I like it as an accent but just not as a whole ring.