It's about two working girls who walk past store windows and play the
game, what would you buy if you had a hundred thousand dollars.  And
one day they see a necklace in the jeweler's and realize they are way
off and the last line  of the story is "what would you do if you had
one million dollars."  It's something like that.  The numbers may be
off, but it's two young working girls from modest means who suddenly
get the realization of what very expensive things cost.

Hello.

I believe that the story is "The Standard of Living" (1941) by Dorothy Parker.

"Dorothy Parker's narrative, The Standard of Living, investigates the
lives of two women and the very shallow friendship the two had formed
with each other.  In the story, the two women, Annabel and Midge, are
very slim women who work as stenographers earning about 18.50 a week. 
The women engage themselves in a very interesting game.  This game is
basically a fantasy game for the two.  The game is based on the
answers of the other to the question of "What would you do with a
million dollars?"
source: Rutgers Journal, cached by Google:
http://216.239.57.104/search?q=cache:Zwqc8Fumw_oJ:pegasus.rutgers.edu/~jmabalot/journalS2000.html+%22+Standard+of+Living%22+%22two+women%22+%22dorothy+parker&hl=en&ie=UTF-8
Shortened URL, courtesy of SnipURL:
http://snipurl.com/7tyd

The story is available in:
Complete Stories
by Dorothy Parker
http://www.amazon.com/exec/obidos/tg/detail/-/0142437212/

--------
search strategy:
"short story" " window shopping" "million dollars"
"dorothy parker" "standard of living"

I hope this helps.

I agree with Juggler. Your story is definitely Dorothy Parker's "The
Standard of Living."

At the end of this story, Annabel and Midge (who have been playfully
discussing what they would do with a million dollars) are confronted
with a necklace which costs $250,000. They realize that a million
dollars won't go as far as they'd thought.

Here is the last paragraph of the story:

"Listen, Annabel," she said. "Look. Suppose there was this terribly
rich person, see? You don't know this person, but this person has seen
you somewhere and wants to do something for you. Well, it's a terribly
old person, see? And so this person dies, just like going to sleep,
and leaves you ten million dollars. Now, what would be the first thing
you'd do?"

Yes, the $250,000 item of jewelry is also mentioned on the page I cited above:

"The two women were surprised to find that the necklace is worth
250,000 dollars, one quarter of the million. Annabel and Midge are
rather disappointed that a quarter of the imaginary money must be
spent to buy a single necklace.   Very much like children do, the
women revise the game raising the money received to 10 million
dollars."
source: Rutgers Journal, cached by Google:
http://216.239.57.104/search?q=cache:Zwqc8Fumw_oJ:pegasus.rutgers.edu/~jmabalot/journalS2000.html+%22+Standard+of+Living%22+%22two+women%22+%22dorothy+parker&hl=en&ie=UTF-8
Shortened URL, courtesy of SnipURL:
http://snipurl.com/7tyd