So buy 10 tapes, label them M-F plus 3 for EOM, and just set the script to append to tape every week, resetting only on the first week of the month. The odds of both the EOM tape and a failing tape being on monday the first are pretty slim. So now, rather than 40+rtapes, you need 15 or so. Whatever you need yo be comfortable.

In other words, ditto on the DLT. We were using DDS3 and just switched to a DLT Library using amanda as the client. We change the tapes once a month. DDS broke way too often to be considered reliable.