Usually a Session person is paid a flat-fee for their time (& talent).
However it really depends on how much the bring to the track. For example, many Dance producers hire different singers to front different songs: invariably, they would be paid a fee plus a small royalty on record sales (commonly referred to as "points"). This royalty would be lower than if they were the actual artist signed to the Record Company because in this instance the Dance Producer is effectively the "artist".