Work is being done on the line on 9th May so there's been a delay in the cheap tickets going on sale. However, they're out now at £12.10 (were previously £46.90) but hurry as there are only 3 left for the 11.38 departure from Edinburgh changing at Perth and 5 for the next train. In this instance it is actually cheaper to book E'burgh through to Inverness rather than separately.
You can get this deal from www.thetrainline.com