Multi-role Missiles

We're thinking of adapting some fleets we came up with years ago in a
home-grown system to FT.  One large difference between the games is that
at least one of the races used multi-role missiles which included (among
other things) a 'lurk' fuse.  Basically this allowed missiles to simply
be 'dropped over the side' where they would function like an FT mine
field.

Any suggestions on how much SML costs should be increased to allow for
multi-role missiles?