Don't risk with leaking fuel on top of a hot engine. Spending 9,200Rs is better than loosing the whole bike in fire.
Personally I don't think there can be that much of rust on its own. This is just too much and unheard for!
Is there a way for water to leak under the frame and sit there after a wash? Is there rust inside the tank too? Are you sure this is genuine Yamaha tank and not replaced before? Frankly it looks like a cheap copy to me made of inferior metal not quality steel like it should be from factory.
These look like 1500-2000 walay tanky tapay for cg125 that used to leak in 6 months. I am pretty sure these were replaced, maybe the bike got damage and the previous owner replaced the tank with these?
Get a new original tank asap and think about what happened later. Don't risk ur whole bike for saving a few Rs.