I heard variations and i'm not sure how secretive its supposed to be, but let's just say mid 50's for the first one and 3~5 circles each thereafter until you learn the 5th and last one.
Note: Thief THROW VOICE's are different from that of bard's (2 of 5 are outright inferior, IMO). You'll likely be disappointed by the first one unless you frequently get into CvC (RP or otherwise) and want to mislead people.
Most bards I've run into charge 10~30 plats each. Some of them if they know you're near your last one they will charge even more, so don't tell them and they wouldn't know otherwise!
There's one bard I know who would offer it in exchange for favors, and another bard I know who would simply do it for free. I don't know if they like to be advertised so I won't, but you might have some luck from asking around IG (that's how I found the latter guy).
"When I grow up, I want to be a soulless monster trapped in a suit of enchanted armor and compelled into violent service for a commercial institution through torturous psychic conditioning." -- Armifer
