Look below for a calculator which you can use, and keep the following things in mind.
A [fixed dh set-broken]~ 160-330k
-You'll get (your hp is probably not very high) ~5-20k xp more per hour realistically, and are paying .25 gp/xp extra (
@80K melee xp/hour, you'll prolly get more), however nmz points if can offset this cost somewhat.
-Also considering time saved (hours over the long run) spending an extra ~20k per hour is less than what you could make in the hours saved (definitely worth it). So by using dh every 5 hours or so, the 1 hour saved means you are profiting (how much money you earn/hour). Definitely worth it.
-consider also, how much you want to be afk. Being in dh means you need to be low hp all the time. going afk means worse xp rates.
I'd recommend entering your stats, pots, gear, etc. to find out for your account specifically. Rates change based on stats, and while assumptions can be made, it's simply best to just spend 5 minutes calculating it.
DPS calculator by Bitterkoekje