Not anywhere near the Marina District, but if you want to be as close to $100 as possible, try the Harbour Ville Hotel (http://www.harbourvillehotel.com/)

I stayed there in 2008 and 2010 and while the room was small, and the shower/hot water setup was strange - it was cheap and had air con in each room (operated by your key in the slot so no leaving on while out)

Free breakfast as well
Bus stop just down the road (on both sides of the road)