Depends which cost function you take into consideration. The size of the block part occupied by a multi-transaction would still be O(n) as are n transaction. So, yes, it could be cheaper by a factor of 2 or so.
A key thing that Nxt *has* over its rivals is fast txs. If I have a tx with 100 outputs and 1 sig vs. 100 outputs and 100 sigs then the latter is going to be slower both in terms of data size and sig verification.
So I think that Nxt should allow such more compact txs in order to provide the "leanest and meanest" tx system in the crypto currency world.