There is a fair bit of info knocking about in the PF forum. However a NPPL(M) license requires a minimum of 25 hrs (in reality it will be more) at £100-£120 a pop, throw in exams and club membership and other bits and bobs and it comes to roughly £4K. Conversion to NPPL(A) I believe is the PPL Aircraft Technical exam, difference training and a general flying test. Currently being advertised over three days for £1K.
Once you start flying 3 axis microlights you may find you do not want to go onto their bigger brothers because they are so much fun. Financially you can fly a syndicate owned Eurostar for about £30-£40 per hour and other more basic types for less. Whatever you decide, just get started and enjoy.