I have no practical experience with the website, but Tripbase appears to be useful in that it lists a section with Cheapest airlines for flights from Medan to Bali, based upon an average historical price.
Tiket2 also looks like it could be of help.
Note that according to a February 17, 2009, post on the site MedanKu.com, there is no direct flight from Medan to Bali, and the recommendation is to fly to Jakarta for a connecting flight. I have no idea if this has since changed. It does not appear so, if this Mar 2, 2011, query on the Lonely Planet Thorn Tree travel forum is accurate: Flight from Bali to Medan. Yes, I recognize that it cites ‘Bali to Medan,’ but the reverse is also discussed.
Medan To Jakarta With Garuda Indonesia B737NG
Uploaded by AVIATIONMANIAC on Jul 25, 2009
GIA189 PK-GEL B737-800