May I ask what the buoy would be used for?
There's really not much to a buoy physically, you could make it from a 44 gallon plastic drum, but there may be requirements to carry a particular topmark, or a radar reflector. or a flashing lamp with a particular flash sequence At any rate the coastal authorities may require approval of the location and may have to include it in their charts.
I was part of a team that maintained a data buoy. It may seem like a simple thing but it was a lot of work: corrosion, biofouling, guano covering the PV panels, damage due to collisions with apparently drunk and blind boat skippers... it required attention every 10 days on average. Calmer and less trafficked temperate waters would have been better, but we don't get to dictate that.
It was that experience that convinced me the future lay in autonomous underwater robots. Mine is here.
There are a few robots out there that can do year-long missions: the wave glider, slocum or similar glider, Argo float come to mind. The Saildrone is a bit new to say how long it will last out there. Of these the Argo float is the cheapest, USD10k I'm told.