Slightly rusted chassis

[ButterflyJayne]

My cat has passed it’s MOT . 
it has a slightly rusted chassis.. apparently very little, no rot, but all the chassis. 
how much would it cost to sand the rust and paint with an anti rust paint 

[TonyHSD]

IMO there is absolutely no way of stopping corrosion once has been started. 
Any attempt may only postpone the end or make it even worse and end to come faster. 
I have an old car as yours and same thing.
The decision was made and it’s simple, just drive as is and once it’s fail the mot for that reason it will go straight to the recycling centre.  
Save myself some cash to purchase a replacement car instead.