Finding data on (estimated) trade cost elasticities of imports
Answering my own question --- although I don't have a a reference per se, I've been told that


if you have no idea at all, -5 is a good ,,first guess``, and
there is a paper (and this is what I don't have a reference for, sadly) where the author worked through all the available estimates and arrived at -4,25 as the ,,best guess``.