TH investigates ROI timelines for building a new GPU mining rig roughly TODAY, and concludes that "it's NOT worth it".
I took a quick peek at that, and I saw one glaring problem... their prices are based upon eBay prices. Frankly, if you're going to start mining, and your methodology is to use eBay as your source of GPUs, you're already starting off on the path to failure.
Although, speaking of eBay, I've been trying to clear out a lot of my old hardware, and I usually check eBay to see the going price. Originally, I didn't see much value for my older, Skylake-based Pentium CPUs, but then I checked again more recently, and... yeah, they're worth more now. I wasn't really sure why until I saw a combo auction... people are using the old Skylake-based Pentiums because they work in 100-series boards, which is what the old mining motherboards used. In other words, you can buy a brand new Alder Lake Pentium for $50, but there are no crazy mining boards for them.
Awkwardly, I delidded my old Pentium G4400 since it wasn't worth much. I was going to use it to practice my relidding proficiency using metal-based TIM, but now that the CPU goes for close to its original price, I just threw it back together with some normal TIM instead. Awkwardly though... I put the IHS on upside down.