Usually, £800 ish would be my limit. However for the last few years this is no longer a simple decision, as it's impacted by mining, crypto and lockdowns.
Effectively, I was paid £321 to take delivery of my 3090.
Bought 3080 in November, for £830
Sold previous GPU (Radeon VII) for £800 on an acution website (ignoring this from calculations for now)
Mined £1200 (profit) from early November to June
Sold 3080 on an auction website (bid style, let the market determine value) for £1350
Bought 3090FE for £1399
There are many in the same category as I, who have effectively had free GPU's this generation.