As requested in the Errata Discussion Thread on that board, I'm redirecting this question here.
I think I've found what may be an error in the current TacOps errata document (version 3.03). It states on p. 111 that the C-bill cost for the Extended LRM-15 should be 218,750. This doesn't seem to fit well into the general ELRM cost progression (see below), but would appear to be a better match for the Enhanced LRM-15, whose C-bill cost is indeed a bit the odd one out:
ELRM-5: 60,000 C-bills
ELRM-10: 200,000 C-bills
ELRM-15: 350,000 C-bills pre-, 218,750 (?) post-errata
ELRM-20: 500,000 C-bills
NLRM-5: 37,500 C-bills
NLRM-10: 125,000 C-bills
NLRM-15: 157,000 C-bills
NLRM-20: 312,500 C-bills
So...typo? Should that erratum refer to the Enhanced LRM-15 instead of the Extended one?