I cant quote u the exact price as I havent been to the scrappers for a while now..... but I suspect you can find either one of these in running condition for about 20k max. Shayad thora sasta ho. I bought a Yamaha SR250 engine..... it too is a single cylinder SOHC 5 speed gearbox about an year ago....... mujhay mint condition may mila tha engine + Carb for 12k from Rachore Line Karachi. I do not think price mein itna farq aya hoga.
As far as the engine mounts are concerned then just like fitting a different engine in a different car you have to change the engine foundations...... same way.... installing a bigger engine in a bike you need some custom made engine mount for the front....... peechay engine to mount kernay kay liye ek hole kerna perta hai for the axle. Its no big deal......
More importantly you have to ensure if the engine can fit absolutely upright under the frame without cutting the frame backbone. I think GS ke frame mein araam say itni jaga hoti hai.
Petrol average wise..... I dont think GS125 will be significantly easier on the pocket..... remember smaller engine does not always mean better efficiency. Agar insaano ki tarhan chalao GS150 ko tow donno same average daingi. If I rev my chonda 70 furiously.... it will give me same average as a CG125......... bottom line ...... Its not just the bike which improves petrol consumption........ its the person twisting the throttle to0.
GS125 flop huwe thi bcoz of its stupid price.....werna bike mein koi masla nahi hai. 90% same spares hain dono ke.