For 200k VND you can get to above 40th floor to have a view (recommended at a sunny day, especially for sunset) or for 250k VND to the same as for 200k AND also above 50th floor to Heineken's brewery. You get a 45min tour, 2 beers and a bottle of water.
Awesome place for sunset! While everyone heads to Promthep Cape, Black Rock remains nice and quiet with occasional tourists. Short hike is required to reach the spot.