2015s don't seem to be holding their value well in the past year, due mostly to massive $5-8K incentives for buy/lease on the new old stock cars in spring/summer of 2017. It's a good time to buy. I don't think they'll drop much more, and may actually retain most of the value since they're possibly the last generation of TDI ever offered in N America.
It's not out of the realm of possibility to buy a nearly pristine used 2015 with under 30K miles for under $15K. I'd call that the sweet spot. Assuming no warranty disqualifying mods or tunes, you'll have an 11-year (from inservice date), 162K mile warranty on the entire fuel system, emissions system, turbo and engine as well.
EDIT: I see you're in Canada. I can't speak confidently on the prices up there.