A room at rental of $300.
A room at a rental fee of $300.
Welcome to the forum. Neither is very natural. For a start, it's not clear whether the rental cost is per day, per week, per month, per year or something else. It's also not a full sentence or at least, without context, it isn't.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.