I also bought two superb professional-scraped plates for £35 but that was very much the exception I would happily have paid £100 for them. The £20 one I bought was in fairly nice nick but has a rust stain from a baccy tin and a couple of other patches. It was actually 8" x 12".
Flatness is what's important with surface plates. They could be badly stained from oil or whatever but if they are flat they still work. Of course, flatness can only be gauged with professional tools or another known flat plate. I reckon a tidy 12" square plate would go for £50 on eBay
The small ones are sub £50. The price rises quite steeply for larger ones and granite ones too.
I suspect that there is the rarity of the objects here too. Lota of small workshop plated and fewer reference plates. The the value of an old one is neat the cost of a new one.
I was looking a few months ago and ended up with a chap 12x8 for £30 i think
A few years ago i had an ex school one on a table stand about 20" square good condition, advertised it for months, nobody interested & in the end scrapped it.
Since it's in Ireland it might be a bit more valuable. I don't know what engineering stuff is like to find over there but I do see some stuff that sells for much more than I would expect.