After Della realizes that she only has 1.87 to buy her husband a gift, she sits down and cries.  Then she gets an idea, she decides to check out the idea of selling her beautiful long hair. Della goes into a shop where hair is bought and is told that she can have 20.00 for her long brown hair.  She makes a quick decision and sells her hair.

"Will you buy my hair?" asked Della.

"I buy hair," said Madame. "Take yer hat off and let's have a sight at the looks of it."

Down rippled the brown cascade.

"Twenty dollars," said Madame, lifting the mass with a practised hand.

"Give it to me quick," said Della." (O Henry)

Keep in mind that the story takes place in the early 20th century, written in 1906, so the value of 20.00 is much greater than what we think of it today.  And, remember that 20.00 would buy a great deal more than it would today.

"The "Dillingham" had been flung to the breeze during a former period of prosperity when its possessor was being paid $30 per week. Now, when the income was shrunk to $20, though, they were thinking seriously of contracting to a modest and unassuming D." (O Henry)

Della's husband, Jim was making an incredible amount of money 30.00 per week, and then his salary was reduced to 20.00.  So if his entire week's pay was 30.00 and then 20.00. imagine how much 20.00 for Della's hair would really be equal to today.

With the 20.00, plus 1.87, she already had, she buys a chain for her husband's watch.

"Twenty-one dollars they took from her for it, and she hurried home with the 87 cents. With that chain on his watch Jim might be properly anxious about the time in any company. Grand as the watch was, he sometimes looked at it on the sly on account of the old leather strap that he used in place of a chain." (o Henry)