I dun think miniDV tape cost should be a factor to change to HDD.
How much does 3-5 tapes cost per wedding ? $30 ?
What is the % of this $30 to what you charge ? I think very very small right ?
Its not the cost of the tape, but rather the time in your NLE.
If your wedding uses 5 tapes, you will need roughly 5 hours just to dump the DV into the PC, should your 5 tapes worth of info can be dump into a HDD by recording instantly, you reduce the time needed for transfering the footages drastically... what needed for 5 hours turns into 1 hour.