Inchyra Blue is that gorgeous brooding teal-grey that goes blue, green or near-black depending on the light — and that complexity is exactly why it's hard to dupe cheaply. Most budget "blue-greys" land flat and lifeless next to it. But there are honest contenders.
Dulux Smokey Slate is your best bet at ΔE 3.2 from the original. That's not a perfect match — anything under ΔE 1 is imperceptible, under 2.5 is very close — but 3.2 is close enough that nobody's going to stand in your hallway with a colour fan calling you out. Its LRV of 12.3 sits right in Inchyra territory, so it'll read with the same depth and moodiness. For a hallway or a feature wall, that's the one I'd reach for.
COAT The Coal Drop comes in at ΔE 5.4 (LRV 14). It's a touch lighter and not as faithful, but COAT's an eco, low-VOC formula with proper modern paint quality — if you care about the green credentials and want something a bit more contemporary, it's worth a sample.
Crown Slate Grey is ΔE 4.5 (LRV 15.9) — the lightest of the three. It loses a bit of Inchyra's drama, so I'd only use it if you actually want a slightly airier version.
The "but what about" here: a low ΔE on a swatch doesn't guarantee a match on your wall. Inchyra Blue is a chameleon, and cheaper paints often have simpler pigment loads that don't shift the same way through the day. Order a sample pot of Smokey Slate and Inchyra itself, paint both on a bit of lining paper, and move it round the room morning and evening. If the cheaper one holds up in your light, you've just saved a tidy sum, mate. If it goes dead in north light where Inchyra still sings — that's your answer too.