Stephen Vaughan/Warner Bros. Entertainment

The last $100 million is the hardest.

The Dark Knight yesterday became only the second movie in Hollywood history to gross $500 million or more. It now stands within $100 million of toppling Titanic as the biggest-ever film at the domestic box office.

Elsewhere, Tropic Thunder made it three wins in a row at the weekend box office, with a $14.3 million four-day, holiday take, per studio estimates today from Exhibitor Relations.

The Dark Knight's estimated Sunday gross of $3.3 million put the Batman movie over the $500 million mark only 45 days into its release, per Box Office Mojo stats.

For the Friday-Monday, Labor Day weekend, the Christopher Nolan film made $11 million. Its overall take now stands at $504.7 million.

Titanic rules the all-time box-office chart with a domestic gross of $601 million.

While The Dark Knight has made its money in lightning-fast time, it is not expected to have enough left in the tank to get to $600 million.

Not that $500 million won't get you pretty far. 

Drilling down the box-office standings:

• Tropic Thunder has now been the No. 1 weekend movie for more weeks (three) than any film besides The Dark Knight (four).
• Despite its run at the top spot, the $90 million-ish comedy has yet to make back its budget. As of today, it's made an estimated $86.6 million overall.
• Dollar for dollar, Pineapple Express (ninth place, $3.5 million Friday-Sunday; $4.5 million Friday-Monday) remains the R-rated comedy hit of the summer. Made for $27 million, it's made $80.9 million at the box office.
  
• The $25 million House Bunny  (fourth place, $8.3 million Friday-Sunday; $10.5 million Friday-Monday) held up well in its second weekend, and brought its cumulative total to $30 million.
• The spoof movie is sputtering. Disaster Movie (seventh place) earned $6.2 million Friday-Sunday, well behind this year's Superhero Movie ($9.5 million) and Meet the Spartans ($18.5 million).
  
• Vin Diesel's box-office career is sputtering, too. Yes, his Babylon A.D. ($9.6 million Friday-Sunday; $12 million Friday-Monday) finished a close second to Tropic Thunder, but its debut was far smaller than that of The Pacifier ($30.6 million) and The Chronicles of Riddick ($24.3 million), to name his two most recent wide releases.
• The new Steve Martin-hatched spy movie, Traitor (fifth place), starring Don Cheadle, actually outgrossed Tropic Thunder, theater for theater. It earned $7.9 million from Friday-Sunday, and $10 million from Friday-Monday.
• In its second weekend, Rainn Wilson's The Rocker played in more theaters than The Dark Knight, something you'd never know by looking at the numbers: $1 million Friday-Sunday; $1.3 million Friday-Monday.
• Hamlet 2's another comedy unable to crack the Top 10. In its second weekend, the critically praised satire broke wide…and flopped: $1.7 million Friday-Sunday; $2.1 million Friday-Monday.
• Debuting at 2,123 theaters, the new comedy College ($2.1 million Friday-Sunday; $2.6 million Friday-Monday) flopped harder.
• The Japanese spaghetti Western Sukiyaki Western Django killed. At one theater, it made $10,236 Friday-Sunday ($14,000 Friday-Monday) for the weekend's highest per-screen average.
  
• Keifer Sutherland's Mirrors ($2.7 million Friday-Sunday; $3.5 million Friday-Monday; $25.5 million overall) and Star Wars: The Clone Wars ($2.7 million Friday-Sunday; $3.5 million Friday-Monday; $30.4 million overall) both fell out of the Top 10 after two weekends.
  
• The first Mummy movie cost about $80 million, and made $155.4 million. The third one, The Mummy: Tomb of the Dragon Emperor ($2.6 million Friday-Sunday; $3.5 milion Friday-Monday) cost about $145 million, and has made $98.7 million so far.  

Here's a recap of the top-grossing weekend films based on Friday-Sunday estimates compiled by Exhibitor Relations:

1. Tropic Thunder, $11.5 million
2. Babylon A.D., $9.6 million
3. The Dark Knight, $8.6 million
4. The House Bunny, $8.3 million
5. Traitor, $7.9 million
   
6. Death Race, $6.3 million
7. Disaster Movie, $6.2 million
8. Mamma Mia! $4.4 million
9. Pineapple Express, $3.5 million
10. Vicky Cristina Barcelona, $2.8 million