This is approx cost of the usual items which need replacing  on a typical  Kawasaki Z1 900.I am assuming the bike you have has no engine problems

4 into 4  mufflers $2600
Brand new painted Tank, sidecovers ,ducktail $2000 ( new not repainted)
Front & rear guard $350 each
Speedo & Tacho restoration $500 ( Z-resto in usa or otto instruments in brisbane)
New complete seat $250
Handle bars $70
rear grabrail $80
side cover & tank badges $150 for the lot
4 indicators & stems $200

Assorted other items( including some bright  zinc plating)  depending on bike allow extra $1750
Average cost of restoring around $8000
Of course this does depend on how fussy you are and does not include cost of  powder coating or respraying in enamel  frame or any Labour cost
Hope this was informative
cheers graham