They're good questions. A headset costs $250, but it can be less. Sometimes witnesses buy their own headsets and are reimbursed up to $250. So it can cost them $50 or $70.
To answer the question about overall budgets and the overall envelope, I'll give you some examples. If we adopted an amount of $41,900, then that amount would be allocated to this study. However, how it would be spent would depend on the variables that were mentioned. We can spend more on witnesses who attend the meeting in person. It doesn't have to be exactly 12 headsets or 18 witnesses participating in person. If we didn't spend all the money, what would remain would go back into the committee's overall envelope. So the money wouldn't be wasted. It would stay with the committee and be used for a future study.
We usually prefer to err on the side of caution by adopting a slightly higher amount, which avoids making supplementary estimates should there be a lack of funds. In such a case, it would be possible to prepare supplementary estimates. By adopting a higher amount, we can put the money back into the committee's overall envelope if we don't use it all, even if it means using it at another time.