I have one, its basically a 400€ phone for 600€. You pay extra for a very long warranty, software updates, repairability and fair(er) production. There is more or less no other option, if you value these things.
If that is worth the price you have to decide for yourself. For me it was, but I understand if people see that differently.
And by its very nature, the fairphone cost is amortized over the long time you won't need to replace it with a brand new phone :-) (although you need to take into account the possibility of replacing an expensive part, which are not cheap indeed but still way cheaper than a new phone anyway)
You're not payingg for just the hardware. You're paying for it being sustainably sourced (as much as possible, or at least as best as available to consumers), and the people manufacturing then getting a fair wage.