Purple seem to be the cheapest, I got a colonie (5-6heads) on a golfball size LR for $15 at a LFS. The blue and red ones are more, $10-15 a head, they grow/bud rather fast, so not many are needed to get a few colonies going.
Cheers,
Josh
PS: I've paid $1 per plyp for zoas, up to $35 a pop, just depends how rare they are.