i depends on what you have to do also actually.
asked around normally when the word overhaul comes to place is they quote $200 - $300.
but this is only replacing of piston, from wad i experience.
but if you have to change conrod but not the piston is $300 - $400
but if u change piston conrod block, i guess getting a new bike is a wiser option.
*they = Unique,Heng moto, Republic moto, brother moto and small shops at Yew tee Industrial park.