This is probably the most difficult bit for most students. Having researched what you plan to do, you should start to get some idea of the costs involved! Most Gap Projects will cost you several thousands of £'s depending on where you are going and for how long.
Remember that the Non profit organisations are generally cheaper! As you start to collect this information, you should start recording it in a Spreadsheet. This will enable you to manipulate your figures so that you can figure out what monies you have and where it is going!
Look at some of the travel guides such as the 'Lonely Planet' series to get an idea how much it will cost you during your travels.
Remember that the more research you do the more accurate your calculations will be. There is nothing worse than running out of money half way through your travels because you had not planned it well enough!