Custom seat with gel insert for rider from P&P seating is £295,
£340 for gel insert for both rider and pillion
You can get a bean bag cushion thing off eBay for about £25 which is decent enough on long motorway journeys but personally I don't like it for town riding as it raises the seat height.
Quite tricky to get pannier fitting kits for the older fzs model now, have to be a hawk on eBay or get some made-
The parts that fit to the pannier boxes (bolt posts and the lock point) can be bought from givi for about £12 a set, so if you have access to welding equipment or a mate that has you can make your own side racks pretty dam cheap, which is what I've done in the past!