As mentioned, if you haven't already got a backrest and highway pegs, I would spend some money here first. These allow you to change your position often on longer rides and help decrease hot spots on the rear and back. How about your handlebars? Are they allowing you to sit in a natural, comfortable position?
No matter how you go, new or reworked seats are expensive. You might want to consider a gel pad or Airhawk seat pad over the stock seat and see if that is a more cost effective solution to your problem.