The price of the 3xxx series CPU's has been gradually increasing again, post Comet Fail Lake launch, you can see this trend if you follow the market daily, you only need to look at the price of the R5 3600 the average price has risen almost 15%, from lows of ~£140 to around £180 if you look at one today.
So six months ago you could have had the same CPU for £40 less, how does this translate into the 4xxx launch? Well the prices seem like they have started to slow down (in terms of price increases) and obviously there is going to be some pent up demand for those waiting for 4xxx, but there isn't loads of stock around like there was with the 1xxx and 2xxx when the 3xxx launched. So it is hard to say the difference this will make, not to mention the 3xxx CPU's are much superior to the previous two generations, so I expect demand to be high on the second user market. The prices for a 3600 as an example may easily fetch £125+ due to the fact they spent a lot of time priced higher recently, if you happened to buy one from here and paid £209 last week, then £125 seems like a big drop, but if you bought at £140 then £125 seems like a great price to get, so it may land somewhere in the middle.
Hows does this fit in with the 4xxx launch, well if AMD pull a blinder and do a £179 6c/12t CPU that is way faster than the 3600, then adding £55 a budget might not sound a lot from a second user 3600 at £125, but that will buy you 16GB of DDR4 3200MHz RAM, or bump you up from a low-mid GPU to the next tier e.g. 1660 Super to RTX 2060. I think you'll see them fetch a reasonable price either way, as I've said in previous threads, AMD don't have the spare 7nm capacity so the stock may not be replenished to any significant levels vs the new CPU's and the price may naturally stay a bit higher due to this.