How much does it cost to deal with the bark is falling off my tree?

The cheap outcome is nothing — an arborist looks, tells you it is a sycamore doing sycamore things, and leaves. Costs climb when the dead face is on the side toward a house or driveway, because deadwood above it must be roped down from a climber or bucket instead of dropped. If decay has entered the trunk, resistance drilling or sonic tomography to measure remaining sound wood is a separate diagnostic fee. Trunk position drives price more than tree size here: a dead face on the target side costs more to work around than a larger tree with a clean stem.