No one knows.  Occasionally some are made available.  Twice I've been offered the 30 I would need for Yoshi.  It would have cost $150 US to get them.

So how much per frag?

lol

SMH!  $150 / 30 frags = $5 / frag

In my experience, they are always offered if you haven't finished a fusion, and yes, it's been $5 a frag, or there about. Occasionally, there have been special events and tournaments where you can win  5-10 frags of various champions. 

5 dollars a fragment? Crikey...

I think they're only offered if you're "close" to completing, though I don't know what they define as "close".

up to 50

50 is correct, though they're not always instant to show up after fusions, etc.

has been awhile since I haven't finished a frag fusion so can't say for sure exactly when they'll show up. if you don't nab 'em immediately they will show up again, and again, and again :)

They should offer 100 frags for $500.  Or would that undercut what whales spend to win them?

Does anyone know definitively - is it possible to win 150 frags and then buy the remaining 50, so you end up with 2 fusion champions?

as far as I can tell: yes. that is what I have heard the approach is from end game PvP clans and I would be surprised if that wasn't how it worked.