To your first question, I don't think it would be possible to calculate a figure. Such a figure would have to somehow distinguish between official money spent as part of the Saudi government budget, much of which would be "dual use" (e.g. is a Saudi religious television show that is intended for a domestic audience but is broadcast on satellite TV to be counted or not?) Then you would also have to add up all the money that comes from Saudi NGOs and QUANGOs. Then you would have to add up all the money that comes from Saudi individuals, presumably with a particular focus on the royal family and other well-connected wealthy persons.

Having done that you would then have to make further determinations about what part of the money involved the spreading of Wahhabism, in particular. For example, during the Afghan Jihad against the soviets, the Saudis matched US funding dollar for dollar. These funds largely went to hardline Islamist groups like that of Gulbuddin Hekmatyar. But does that count as "spreading Wahhabism?" Was the US "spreading Wahhabism" in doing so as well? Is all Saudi overseas development aid in some sense to be regarded as money spent towards encouraging Wahhabism?

I suspect an answer in hundreds of billions range is plausible but I don't think it would be possible to get a precise answer.

I'd also be curious to knowing how much money the US has spent spreading capitalist ideology.