Hi Emma, your first week will be busy and there will be lots of people around (well in Playa D'en Bossa area anyway, don't know what San An's like at the end of the season) until Space and DC10 closing (1st and 2nd October). After that it will be quiet and you will have some nice quiet beaches to relax on!
Last year the weather wasn't great in September, but I've been before when it's been lovely.
Very difficult to say how much money you will need as it depends on how what kind of holiday you want, how many big club nights you want to go to etc.