Most expensive is probably prescription tretinoin as I see a dermatologist but my insurance pays for that (I'm not in the UK) so not sure how much it actually costs and I see the dermatologist for other issues primarily. I'm guessing it runs into the several hundreds and the cream itself is around €20.
OTC most expensive was probably medik8 r-retinoate, €169 iirc and it is a really great product if you're in that same rare group of people like me who can't tolerate the conversion process of traditional retinol very well but has no issue with the active ingredient itself.
I've also tried Skinceuticals CE ferulic (€134 IIRC), worth the hype in my experience but not the money when there's so many copycat versions out there, Timeless is my preferred one at a fraction of the price.
I also tried Vintners Daughter Active Botanical serum (I had a deluxe sample), it's also stupidly expensive, £175, and that would be a waste of money IME it's just a very nice pampering product that feels amazing to use with very good marketing. You sort of look a bit brighter in the morning but nothing that justifies the price tag and all those essential oils aren't really good for skin anyhow...
Other products on this thread I've used, Bioeffect does seem to do something and it works quicker than any other serum I've ever tried but I was relatively young when I tried it (mid to late twenties) and I think the full potential was a bit lost on me. I've been a user of the Verso eye serum for 5 years pushed Client into buying it and absolutely swear by it. I've never paid more than €60 for it though, and it's twice the size of a normal eye product so actually doesn't work out to bad as it lasts really long (well over a year).