Is adding an expensive tier nobody buys, so the middle one looks reasonable, a dark pattern or just pricing?

The two effects you are leaning on are well documented and worth naming separately, because they are not equally defensible.

Anchoring is that people judge a price against whatever numbers are nearby. A high option shifts the whole perceived scale, so the middle plan reads as cheaper than it would alone.

The compromise effect is that when uncertain, people avoid extremes and choose the middle. Adding an option above your target plan moves that plan from being the top choice to being the sensible one.

Both are real, both are used everywhere, and neither requires deceiving anybody. The important distinction is whether the extra option is a genuine offer. A real plan that a real customer would sometimes buy is a product decision. A plan constructed so that nobody could rationally choose it — deliberately worse value at a higher price, existing only to be rejected — is a decoy, and that is where most people put the line.

Worth adding what actually is a dark pattern in pricing, since the contrast makes the line clearer:

• Hidden costs revealed only at the final step of checkout.
• A pre-ticked upsell the user has to notice and remove.
• Making downgrade or cancellation hard while upgrade is one click.
• A struck-through "was" price that was never charged.
• A countdown timer that resets when you reload.

Every one of those either conceals information or exploits asymmetric effort. Ordering three honest plans on a page does neither — the customer can see all of them, compare them and pick any one.

That is the useful boundary: a dark pattern interferes with the customer's ability to decide. Framing changes how the decision feels while leaving the ability completely intact.

The test I use, and it has held up: would I be comfortable if a customer bought the expensive plan?

If the answer is yes — it is a real plan, it delivers real value, some customers genuinely want it and are well served — then the tier is honest and its effect on the perception of the others is a side effect of existing.

If the answer is "nobody should ever buy that", you have built a prop. Sooner or later somebody will buy it, because somebody always does, and you will have taken money for something you designed to be a bad deal.

That one question separates almost every real case I have seen, and it does not require deciding anything about psychology.

One practical observation from having watched this go wrong: a top tier that is wildly out of proportion can damage trust rather than anchor.

If the plans run 10, 30, 90 and then 2000, the last number does not make 90 look reasonable — it makes people wonder what the real price is and whether they are being played. The anchoring effect has a range beyond which it turns into suspicion.

In the cases I have seen, the useful top tier is somewhere around two to four times the target plan and is clearly aimed at an identifiable kind of customer. Beyond that you are not anchoring, you are performing.