The entire point of the month to month fee is to get them to sign back into a new contract and tie them into another year of rent. The higher the better to a certain extent. Pushing it too high could result in some "you are trying to screw me over" type feelings from your residents and they may leave with a bad taste. With that being said, the residents that get all the way to not signed a new lease, not put in a notice, and are now being charged month to month fees are going to move out anyways or just needed one more month until they are getting a house, new place, etc. Very small percentage gets to that point and pays for a month like that then signs a new lease to stay. Most of these renters that get to this point simply are charged a prorated amount of the month to month fee due to a late move out, late notice, or late signing of a renewal. If they do pay that fee for a full month then stay, they usually had no where else to go, rent was still more expensive elsewhere due to the loss on lease they have with you and they thought otherwise, or they could afford the higher price of the fee and took their time signing a new lease. Wouldn't that be nice to be able to do!
Of course great costumer service in and out of the office is always the carrot to help avoid using the stick of month to month fees to get them to renew.