Helium is comparatively expensive, but you don't expend it as part of normal flight and it doesn't migrate very quickly through modern materials. A bigger problem than loss is fouling - you have to pump out the helium and purify it every so often (I think Zeppelin does it every six months).

Hydrogen is close to free.

~$2 per 10kgs of lift if my internet calculations are correct.

But I guess if helium isn't lost at a great rate, it might be a cheap part of the whole setup.