I asked myself the same question and snooped around for a few weeks a while ago. Didn't find anything that is a lot cheaper with the same quality. I even searched Aliexpress and the spin-offs are not really cheaper enough to justify the potential degradation of quality (I'm not saying they are of poor quality, just that I don't know).

I think Raspberry Pi enjoys from economies of scale so much that it doesn't even has boxing day deals. It's really cheap enough considering a 4GB Pi4 module only costs $72.95 in CAD. If you want there are clearance sales on pishop.ca:

https://www.pishop.ca/product-category/clearance/?sort=price...

Just a note that those clearance get cleared really quickly. I'm also thinking about getting a few second-hand Pi for a cluster I'm trying to build (which can cut down the cost further), but the thing is it's difficult to find reliable used Pi in local market.