You will find the most affordable options in North Brunswick - its a more middle class community.
East Brunswick is more affluent an area then North Brunswick but still has a decent amount of income ranges - East Brunswick is HUGE. So you will probably find lots of rentals there.
Princeton is an expensive place to live. There are some areas of subsidized housing but for affordable apartments you will be competing with college students looking to live off-campus. The Boro has some more affordable areas but some of those locations are pretty rundown........
You can check out this website to get an idea of rental costs but you'll probably find more on Realtor.com - this site is better for properties for sale -
GSMLS ....... North and East Brunswick are Middlesex County and Princeton is Mercer County. Good luck. PS - lots of places have a Princeton mailing address that are NOT part of the 'real' Princeton, if that makes any sense.........