The helium is generally the biggest cost for a launch. You generally need welding grade helium instead of the party balloon variety which costs more. The price fluctuates a lot because of some complicated economics related to natural gas production, but getting that high can easily set you back $400.
If I recall rightly, hydrogen leaks out of the balloon so fast because of the change in pressure at high altitudes. Not a problem at lower ones, but way up I seem to recall that it is.
