Having had a similar thing done, the reason for this is that Apple will not replace a single key on the keyboard once a laptop is shipped in for repair.
In some stores, techs may try to re-seat the existing key, and in very rare cases have replacement keys to utilize in-store, but usually they end up wanting to ship out for a full replacement top case.
So you are not paying for a key, you are paying for a new Apple keyboard (which sadly, will inevitably develop the same key issues as the design is not fixed.)
It's not just 'a space bar fix'. It's a key on a highly specialized keyboard. A MBP is the same price as N Chrome books but they aren't in the same ballpark in terms of functionality.
I'm sorry, but $257 for a spacebar repair is simply absurd. To put it in perspective, $72 more would get you a base-spec iPad.
I don't know about you, but a single chiclet key and a full-fledged tablet computing device aren't in the same ballpark in terms of functionality, either.
You'd probably be surprised to find out how much it costs to replace one 2x4 in your house then. It's just a 2x4, right? How much could it cost?
Until you factor in the cost of tearing out the drywall, putting up a temporary support jack, tearing out the 2x4, putting the new one in (plus the cost of nails, of course), then running the electrical cables back through the new board, then replacing the drywall, and then of course painting the wall again.
But, you say, but you could save the extra cost and expense and labor by just not having drywall covering your 2x4s. That's certainly an option, and I'm sure some homeowners choose that.
But no, it's just a 2x4. It should be cheap, right? But when you add it all up, just another couple of dollars would buy an entire new wall!
In some stores, techs may try to re-seat the existing key, and in very rare cases have replacement keys to utilize in-store, but usually they end up wanting to ship out for a full replacement top case.
So you are not paying for a key, you are paying for a new Apple keyboard (which sadly, will inevitably develop the same key issues as the design is not fixed.)