For comparison: you get 1 shadestone from shadownman in about 10-15 minutes, and 1 fairy dust in about 1-2 hours
Farming shadestones will also give you lot of XP and other useful drops
considering all things, twice the drop rate of fairy dust and reducing the cost to about 5 fairy dust might actually be reasonable
considering that fairy staff is a temporary weapon until you get e.g. voidstaff
the main reason why it takes so long to get fairy dust is that most of the times only gremlins will appear, and if you are solo, waiting for 1 respawn every 30s takes really long...