Billet is for more expensive travel/admission as you point out. Ticket, pronounced as you illustrate tee-kay, is for inexpensive passage such as for the bus. Billet d'avion, billet de train, billet (or ticket) d'entrée (admission ticket)...

What is the purpose of a billet?

A billet is a section of metal used for rolling into bars, rods and sections. It can be produced with ingots or directly by continuous casting. Billets are used as raw materials or feedstock in extrusion, forging, rolling and other metal-processing operations.

How do you use billet in a sentence?

Billet sentence example

When the CRE dismissed me I thankfully set off for our new billet , arriving about midnight. The rest of us followed on Friday relying on our mates to have blagged a good billet . The string is wound once around the billet and held taut by the thumb inserted in the looped end.