The problem is how much would that even cost him? How would he know in advance if that was even going to give him any useful dirt? There was already a huge cost to save someone's grade season 1, and on his own he didn't have enough to do that, he needed help from someone else who had saved up an incredible amount of points.
And at no point do I feel anyone in the anime has been given plot armor after the first two seasons.
I think he had 90k plus points when he tried to struck the deal. Left the clasroom conspicuous for Horikita. Knew Horikita saved up her points in advance and played her drive to do good if it aligns with her personal objectives. Ayano could have just be covert about striking that deal which struck me odd that he made his intent obvious when Sudo's expulsion. Seeing the scene first time made me think he knew Horikita was going to be there and help as planned.
How much would anything cost him? He can ask if he wanted. Just as how he could ask how much would it cost to remove the restriction on their cellphones sim. He didnt know the value but asking wasnt a problem and covertly.