CamelCamelCamel as noted above has historic prices for components which isn't that useful.
CPU historic prices are almost irrelevant, as they are basically in set price brackets, and the only time it's cheaper is if it's on a sale/offer, which you can't predict. Intel CPU prices have a very specific cadence which has been consistent for something like 5+ years now. Unless you get a Microcenter deal then it will be the price it is.
RAM is very volatile and historic trends have no relation to future prices.
SSDs are on a somewhat consistent downward cycle so the longer you wait the better the deal you will get pretty much guaranteed, again not much point in looking at past prices, since they won't be relevant for the future. Sales will be sales and can't really be predicted.
GPUs are probably the most volatile but depend primarily on new products being introduced more than anything else, which often shifts pricing, but price cuts and movements can't really be predicted.
Motherboards, PSUs, cases etc are all fairly immovable and unless there's a new chipset around the corner all three will be fairly static.
So basically, there's no point in looking at historic prices and you just need to keep an eye on hot deal forums for the parts which will have sales, or buy whenever you are able the parts which won't change in price.