In the past you could pay something like 30 USD to enter and roam around the lobby. Now, you need to be staying at the hotel, or have a breakfast/lunch/ dinner reservation, or what I think is the cheapest option, is to go for high tea, which costed over 100 USD last I checked.

The brunch at the Burj is currently about USD 135 excl. alcohol.

The Burj Khalifa has recently opened the world's highest restaurant (though I think EK First is probably higher), and the minimum spend per head is USD 120.