I also just went to realtor.com and did a search for you and didn't come up with anything in your price range, but did find one for $ 725.00 and it is a 3 bedroom, 1 bath built in 1965 in Chatham County.
I am sorry I didn't find more, but try to find an agent in that area that specializes in Rental Properties.
They may have more resources for you.
Keep looking on realtor.com
I just did a search for rental property in the First Multiple Listing Service based on your requested criteria and no property came up.
I then did a search for all rental property in Port Wentworth...still nothing.
I'm guessing there are homes for rent, but they just aren't listed in FMLS. My suggestion would be to link up with a local apartment finding service or a local Realtor in the area that can either help, or at least point you in the right direction.
To find a Realtor in the area go to http://www.kw.com and search for an agent.
Hope that helps!