Considering it is a challenging thing to do, it should be taken into account, I'd say if the task gave around twenty thousand experience points it would be a good start. I wouldn't oppose more than that, just a base value of what I think is the lowest it should ever be, due to the scale of the endeavour. Frankly with the ammount of slayer xp that is gained on a normal task, i'd say a lot more than that actually. If you equal it to say a bloodveld task for example.
As for the points, that's a hard one. I'd maybe suggest a hundred? Perhaps? Just giving opinions and thoughts on it.
Kind Regards
Tyanx