cheapest would be to get a stock HD650 cable from sennheiser (should be around $15 for the cable and shipping) or use the one you already have, then get a pair of XLR plugs of the type you need (3-pin male if you want to use it with a balanced headphone amp, 3-pin female if you want to use it straight from the dac1. you can make a convertor to switch between the two), and recable it yourself or have a friend do it for you.
otherwise you can get a balanced cable from Zu Cables (the balanced version is IIRC $25 or so more than the standard one, ask them when ordering), Moon Audio (prices listed on their site), or Cardas (they only offer one to connect to amps not sources). DIYers can also make one for you....
for costs it would be around $5 if you go with the cheapest method (use your current cable so the cost is only for the new plugs), to a few hundred (depending on length and other factors) if you go with the aftermarket cable manufacturers.