You can buy Lux with 5 or 6 gold.

Happened to me twice now.

Basically, if you have a in shop and only 5 or 6 gold you can still buy her if you click on her (even though the portrait is greyed out).

Edit: Here's some pictures: https://imgur.com/a/vDYY30b

Apprently, you can't just click on the portrait to buy her. You have to drag the portrait up. Works immediately after I drag her portrait to my bench and release it.