Andrias, it is generally quite expensive to fish Sumba and there are not many trips. It is bloody brilliant though both to the north (as this trip) and areas south of the island.
Elsewhere in East NT:
Komodo, Rinca and other parts of west/north/northwest Flores are easier and cheaper and the fishing still good. A Google search will find you prices. Normal is to fly in to Labuhan Bajo.
Southeast Lombok and Sumbawa is often approached out of Bali either on a simple liveaboard or staying onshore in the Kuta Lombok area. Good GT popping and some excellent deep jigging. Trips offered here by the better Bali-based operators are quite popular.
In the far east, Alor is by some reports not as good as it used to be, but others dispute that. I can't say first hand but am going this November for the first time. Alor is known for big black GTs and notorious for big Dogtooths wrecking your gear
. It is not too hard to find good charters at the right time of the year in Alor. Most people fly in to Kalahabi via Kupang and meet the boat at Kalahabi harbour.
I hope that helps demystify a bit.