I feel like with excavations being so cheap that you can literally pay for them in a for a week (with some leftover) with just 1 dig patch run through that having any cheaper way of making of doing them would just be too good not to do when your doing excavations. You could make so much money off them by buying this pass and doing exca's everyday (not counting pokemon you catch along the way)
I personally feel like excavations are very well balanced with profitable items/rare poke you can find, that adding something that just makes them better would ruin it.