Man, you guys are so lucky in the States. Paying under $5 per attachment. The prices between Canadian and American amazon is astounding sometimes.
Regardless, I have managed to amount a pretty sizeable collection of attachments and wands. I have some toys that I call "jackpot" toys because he likes them so much- when he plays with them, you can tell he's exerting 100% energy. I have some other attachments that he initially liked (like Da Bee) but now has zero interest in. There are some toys that he likes occasionally, so maybe 30% of the time that I bring it out, he shows good enough interest to finish an entire play session with it, but most times he will just ignore it.
If he loses interest within the first few minutes, I know he's either A) not in the mood to play, B) doesn't like the attachment I've chosen. And man oh man, he's a picky little bugger. Even the "jackpot" toys can become boring to him, in time, so it is a real challenge.
My advice to you would be to try and get a few (like 3 or 4) different kinds of attachments, if you haven't already, and see if he goes for any of them. Try things with feathers or fur- I've gotten good results with those. I had one that was a fur ball with some leather strips attached that he used to really like too- before it got destroyed.
Also instead of buying stuff- try some simpler things from home. Crunched up balls of paper, or a chips bag with a knot in it, or a long string, or anything (a sock with a knot in it) , and you can throw them up and down the stairs and see if he goes for it. Drag the object on the floor to make a sound so you catch his attention and then throw it up the stairs.
PS:
This is one of Miko's "jackpot" toys: (word of caution though: be diligent about not letting them eat the feathers)
https://www.amazon.com/PURRfect-Cat-Toy ... ct+cat+toy