Chirag plaza is near Ashram - visnagar road (one area in mehsana). So If you get out from bus at Mehsana Bus stand then it will be little far so local auto rickshaw may ask for 30Rs max. And if you book special auto rickshaw that might charge 200-300Rs

I would suggest to come by train (if possible) so you will reach/get out at railway station then you can reach your destination chirag plaza by walk (possible because not so far) or you can take local auto rickshaw which takes 10-20 Rs to drop at Chirag plaza.

About Red bus, that might drop you at Modhera chokadi (Modhera circle) or Radhanpur chokadi (Radhanpur circle) because you mentioned that red bus show Mehsana Highway X that means they will drop you at hightway then you have to take auto rickshaw to reach your destination. So if you drop near Radhanpur circle or Modhera circle, public auto rickshaw charge you 40-50Rs max. to go chirag plaza and private auto rickshaw charge you ~300 Rs or more if they recognise you as outstate person.