Just your dad, and not his old room mate? $250, max, with no utilities extra.
That way, he feels like he's paying something (and it's about half what he paid before, with 2 people), so he won't think of it as a handout. Otherwise, if it's with the roommate, then make it $50 more.
Let's face it, it's your DAD. If he balks, it's either because he thinks it's too low (which then you can tell him that he can do a chore or two to make up for it), or it's too high (which then you tell him he can work some off doing a chore or two). Get him moved in, and when he asks what the chores are, tell him it includes relaxing, enjoying the company/grandkids (if he has any?). :biggrin: