Getting ON the riverwalk for less than 80 ain't gonna happen at any place you want to stay at. My folks stayed at a crappy place where the river had been DRAINED and they still had to pay 150 a night. You might be able to sneak in on the street side for 80, but then why bother? If you are really on such a tight budget just crash at an outlier hotel and drive in. It's not like you can drink anyway
BTW, who are you coming with? Family, friends, or a chick? You definitely do not want to take a girl to a fleabag motel on the cheap (well, for more than an hour anyway
)