If Martin's model is to be believed. How low can we expect Bitcoin to go? and by When? Is Under $100 in Spring of '16 a safe bet?
I think bitcoin does not go low, he doesnt state anything about bitcoin
Dollar going high is pretty much the same as bitcoin going low, all else being equal.
On what basis?
Because the main usage of Bitcoin is speculation and it frequently trades in line with commodities (and likewise inverse with the dollar) as others have pointed out here.
That's not a fixed relationship, but it is a good baseline for the short term. Even the Bitcoin peak in 2013 coincides with a relatively low in the dollar. Although gold peaked a bit earlier, in 2012, both Bitcoin and gold have collapsed as the dollar strengthened in 2014 and 2015.