For me it's about my running costs and expenses. Trains and hotels are so dear these days that if I go to an expensive city like London I absolutely have to raise my prices just to make it worthwhile. I do check what other people's rates are to give me an idea of how many others in the area are charging what I'm hoping to charge (if that makes sense!)
If it's quiet I might put phone chat on to try and drum up interest rather than lowering prices but I guess there's no harm in trial and error!

yes same I always put my London prices a bit higher because hotel costs, expenses, etc. are mad these days, I think London is a bit particular as all girls there (if they tour) seem to raise their rates when in London but I'm also really wondering whether I should adjust my rates in other regions (London not included) because there seem to be quite a few variations between regions in terms of how much clients are willing to spend (I always feel that clients from up North clients are generally willing to spend more than those from the South) but obviously it varies