For the old vanities, I'd assume it would be 25 plat so people wouldn't complain about the old price (If I recall correctly, they were 15 plat last season but that was before vanities were tradable).
For Snowball, I'd assume 15 plat since I think thats what it was last year.
If new vanities are released (which there is a good chance) then I'd guess 50 plat each like the halloween vanities.