As it has already been stated, unfortunately it just doesn't work like that. There are many factors that need to be considered which includes things such as whether your connection is hard wired or wireless, if you are on cable or ADSL, where you are are downloading from/uploading to etc.
In an ideal world we would all be getting the speeds we pay for but life is life. ISPs are always careful how they word their advertising to ensure they prevent any lawsuits coming their way. For example, if you're paying for a 24MB ADSL service, you'll find your ISP will say "you will receive a connection up to 24MB".