A set of shells, piston rings, liners and the rest of it isn't generally expensive per piece. (30 quid seems a fair bit, for a four pot, to me, for the shells).
The real cost is in removing the engine, taking the sump / head off, and putting it all back together after replacing the bearings. If you do this yourself, the headline cost is actually pretty low. Price up your own hourly labour at a reasonable level and it becomes expensive (maybe).
But of course, there's a fair chance you didn't tighten something up properly and the block is damaged by a rod or the like. With a rally / race engine that you routinely rebuild and when you have spare blocks / heads to play with it's all about investing time in your hobby or the fact that you have engineers you're paying (formula one and the like) and they may as well be working as watching.
Liners require a press, well - that depends on the engine, but bearings are a spanner / socket set / hands replacement. In that sense an engine rebuild is not a major issue. A garage that "rebuilds" your car engine and gives it a warranty is a *quite* different (and scary) pricing model. I'm not aware of many in between options.