It depends on model and condition, but the market for Northgate keyboards has been pretty solid for a long time. The top of the line, for me, is the Ultra, all 119 keys, not to be confused with the Ultra T with a mere 115, or the Ultra Plus, oddly named because it has fewer keys than its Ultra siblings (it only has function keys on the side, not across the top of the board).
I've never typed on a blue Alps, but I got my first Northgate back in the early 1990s. I have three currently and have to resist the occasional temptation to buy more. I have not yet found a switch I like better.