Well we can't jump to a conclusion so easily...
Yes the work is required, but it depends how much work is required on ur engine. This can only be checked once the engine is opened.
Point to be noted...
Only ur engine head will be opened most probably so dont be worried about it.
Smoke in silencer can be due to worn out valves/valve seals/rings/piston... Any one or all of these - so find an honest mechanic and sit on his head as he does the work!
If a complete general overhaul is required and ur engine is done - means completed its life( Yama 4's engine should work fine for up to 75,000Kms before it requires an overhaul if kept with care thus i doubt about the problem being in the rings + piston)
then the cost is going to be immense! - At least at least 10,000Rs.. put them aside.
If complete engine has to be opened and connecting rod and its bearing is also changed then put aside up to 15,000Rs.
Since genuine Yamaha parts are really really expensive!!