If the belts have been done (and your sure its been done!!) your half way there. If they havent been done then its a good opportunity to get the cams out and check the rockers. Has the bike got any history? If it hasnt then i would budget around £1000 for a 12000 mile service plus a few rockers that its bound to need. You might well get away with less than this but i bought an ST4 which is the same engine and had the same mileage on as yours, nelly gave it a service which cost me over £600 including three rockers.