Della had exactly $1.87 to start with -- at the beginning of the story.  We do not know exactly what she has in mind -- in fact we do not know for sure if she had any particular thing in mind at all.

All we know is that it had to be something special.  The quote is that it should be

something nice for him. Something fine and rare and sterling--something just a little bit near to being worthy of the honor of being owned by Jim.

As it turns out, what she buys him is a chain to put his watch on (this was back in the days of pocket watches).  But I think it's pretty clear that she didn't know that was what she was going to get because she has to go "ransack" the stores after she gets some money.