There are 9 squares involved with the 7, so 480"9=471 other squares. These other squares contain the 92 other mines. So the number of grids with a 7 at a particular spot is 8(47192). That is out of a total of (480 choose 99) different grids.

We have 492556=125244 ways for an easy grid to have an 8 somewhere. Out of the 1.88 trillion total easy grids, this gives a probability of about 610"8. So, very rare indeed!