According to my point of view Noida is one of the best developed city in India .So if you want to invest in property then no doubt you will get benifit..
Now, Noida is one of well known established area in India. So, if you are asking about the cheapest sector in Noida, its not applicable. It is meant for developers, where cheapest word does not exists.
It is difficult to says that which is cheapest seactor in noida because Noida is one of the fast developed city in India. But as per the my opinion, sector 19 is the cheapest sector in noida at this time.
Noida is a nice city in Delhi NCR to investment because it a industrial and residential approved area. Numbers of real estate builders are working in Noida and Greater Noida area Like Ajnara ajnara.co/AjnaraSportsCity/ sports city is good place to investment. It has 4BHK villas and 2 , 3 BHK flats apartments.