MMs are not terribly rare, nor expensive, generally.
Like Hursti and liege mentioned, there are some expensive ones, but the majority are pretty easy to come by.
I agree with them that the Transports are expensive - Missile Launcher, Cannon Transport, Tanker Transport. Battlefield Headquarters is also pretty difficult to get complete.
The combiners, "squads" are quite expensive MOSC, arn US$90 thereabouts and about $30+ to $50 loose and yes, Missile Master is almost always missing the damn missile.
On the Japanese side, obviously all the gestalts are not cheap - usually around US$250 MIB. Don't see much of these loose. There is the reissue option with these though.
Then there are the rare Japanese recolours that are very expensive - eg recolour of Sky Hopper, Groundshaker. Individual releases like Rabbicrater (came with the Zone video tape).
And if you really want to complete the MMs, you'll need all those that only came with the main Zone figures (eg:
Metrobomb, who only came with Metrotitan; the MMs that came with
Grandus,
Sky Garry,
Dai Atlas,
Sonic Bomber and
Road Fire the latter 5 are all distinct new molds and not recolours of anything).
Let the hunting begin!
