here at PVR we are just going through the whole licensing process at the moment. In the UK the organisations involved are PRS and PPL and any type of station needs a license from both. As Keybasher says, it's entirely dependent on what type of station you have. There are three main categories here, those that are broadcast over the air commercially, over the air community and hospital stations (ie non-commercial) and online stations. Of course some are on more than one platform so generally need to be licensed for each.
We are just starting as online only, so technically we are 'webcasting'. If you're just a linear online service, with no on-demand content and no user controlable replay it's much cheaper than if you also do either of those things. Linear streamers also have to follow certain other rules such as how often a song by a given artist can be played. Assuming all those rules can be followed then the price for the licenses currently comes to around £300 in total as a starting point. However you have to keep track on the number of tracks listened to per hour. The figure is worked out by taking the average number of music tracks played per hour and the total number of hours listened to. So 1 listener tuned in for 10 hours is the same as 10 listeners tuned in for 1 hour or 60 listeners tuned in for 1 minute each. So you then have the total number of hours streamed which you multiply by the average music tracks per hour. If you go above certain thresholds you have to pay more. However if you do go above the thresholds then you should be able to get enough advertising to more than cover the increased costs.
So the basic answer is to start up the licence for online only in the UK is about £300.
I have to say I've spoken to both PRS and PPL and they both have really good staff who are only a phone call away and always happy to answer any questions.