What is the value of Liza’s offer, percentage-wise? What is this telling us about her? (Act 2)

Liza is willing to pay as much as a shilling an hour.

You know, Pickering, if you consider a shilling, not as a simple shilling, but as a percentage of this girl's income, it works out as fully equivalent to sixty or seventy guineas from a millionaire.

PICKERING. How so?

HIGGINS. Figure it out. A millionaire has about 150 pounds a day. She earns about half-a-crown.

LIZA [haughtily] Who told you I only—

HIGGINS [continuing] She offers me two-fifths of her day's income for a lesson. Two-fifths of a millionaire's income for a day would be somewhere about 60 pounds. It's handsome.