the whole cost depends on how long you need to complete the course. 45 hrs is the minimum, most take a little longer, mine was 50 hours over about 14 months. if you do well enough to pas in 45 it will obviously cost less than 65.
the grass is generally good at WW if a little bumpy in places. it is hardly ever closed due to waterlogging, and they have 3 fairly long runways. if denham is better for you by all means go there, but my advice would be to pay as you go. you then have the option to change schools if it suits you, and your money is earning you interest in your bank rather than making you nothing in the schools bank, and its not at risk (unless its in northern rock - no, lets not go there!).