Making shadow lairs too cheap to enter as a solo player makes them too farmable.

Making shadow lairs too expensive to enter as a solo player (as they are now) ruins the fun.

Scale the key price in a way where it costs 450e X the amount of people in the party.

Payout wouldn't be touched at all, only accessibility.

A key of a higher tier can function as a key of a lower tier.

I'm not sure how you would implement this, but this is my take on it:

Introduce 3 new keys into the supply depot that open their respectively sized parties' elevators.

1 person shadow key: 450e
2 person shadow key: 900e
3 person shadow key: 1350e
4 person shadow key: 1800e

Note that any pre-existing shadow keys or shadow keys obtained from forge or lockboxes would be 4 person shadow keys.

It's getting harder to find SL parties because not all 4 people are willing to spend up to 2 hours in an insanely hard dungeon who's payout wasn't touched.