You pay for a "run", not a fixed amount of data. Billing is not determined by the output, in other words you don't pay by the megabase. Billing is also not dependent upon performance. We are an at-cost not-for-profit facility. Running your sample uses reagents and supplies so you are billed for the sample/run.
If there is a failure due to reagents or instrument performance we will re-run that sample again at no additional charge. If the failure was due to library construction issues, e.g. you used non-modified home-made primers, or there was some other problem related to the sample, you pay for any re-runs (after the problem is fixed, hopefully). We will do our best to help with troubleshooting.