I'd also add that as discussed in other threads in the Forum, the Church likely intentionally avoids providing that amount, to avoid having families try and "prepay-in-full". Once donations are made, including for full-time missionaries, that money cannot be recovered. So if a missionary unexpectedly returns home early, all donated money remains with the Church and not the missionary. Likewise, there's no way for the Church to know if others may also decide to contribute to that missionary's account, and pre-payment can lead to over-payment.
You may have known that, so just mentioning. If the families are desiring to know how much will be required for completion of the full-time mission, a simple calculation of
"current balance" + ("months remaining" x "amount per month")
should give a reasonable estimate.
Often families right near the end of a child's mission want to know exactly "how much is left". I can't remember what the current rule is for last-month of missions (does the account go live the month the missionary starts and there's no payment the last month? Or the opposite? Or some other calculation process?). I'm sure others in the Forum know those details.