Depends where you are coming from. For those of us on older systems, factor in an NVMe SSD and DDR4 ram. Add a decent aftermarket CPU cooler on top of that too. So I'm facing more like £1k, which is getting kinda pricey. And it's not like there are any decent new games to take advantage of all this upgrading... (enter fanboys who claim they spend all day "encoding, etc." so "need the extra cores"..)
I mean that argument can be levied at any platform when you are upgrading from older tech. That's not specific to Ryzen 3. In addition you can easily go for the cheaper 3600X/3700X parts depending on requirements.
I find the arguments about the motherboards a bit bizarre. There is going to be a plethora of choice for all budgets on X570 so honing in on the extreme motherboards seems a bit odd when computing budget/price ranges. B550 will launch later which will bring even more choice and there is also the option of going for X470/B450 boards as well if you do not want PCIE 4.0 so it can be as cheap or as expensive as you want.