I don't believe for a minute that brakes are cheaper than time. Ryanair and most other airlines pad out scheduled time to give them a chance to catch up over most small operational delays.
The excessive use of brakes is far too often associated with the "rush rush rush" syndrome common amongst many carriers, esp I suspect locos and is employed reflexively by many crews (along with M.80/320) even when ahead of the game.
Most of the time , in my experience, it's utterly pointless and indefensible.