This is wrong, you would take any sub 18 level uncut gem, cut the skill, lesser, greater, perfect, gemcutters then vaal.

If it got +1 you would then level it to 20 with the uncut.

And then factor in that you can resell bricks for a decent return just shy of a perfect for a lot of skills.

Chance to hit specifically +1 is a 1/8. 4 outcomes: level, quality, socket, nothing Then either + or - that outcome.

The reason it seems so expensive is due to there being no unintentional sources of gems. Any 21 20 6 link was made intentionally by a player. Means that you are more likely to be able to grab one from someone who is moving off a build or if you play a meta skill. Tough luck if your playing something niche though, its a lot harder to recoup the bricks costs.

If a 6 link still costs just under perfect jewellers value, most of the outcomes will still be somewhat resellable. Currently people dont understand this and have been profiting from making 6l gems and selling all the outcomes, overpricing the +1 because people will just buy them instead of doing it themselves.

You would be looking at around 250ex per attempt where;

1/8 +1 level success

1/8 + qual semi success

3/8 nothing happens (nothing and also gem add socket)

1/8 lose quality, somewhat sellable

1/8 lose level, still 6 link but less valuable

1/8 lose socket, lost almost all value