[The_Return]

...the ending is indentical to your description, actually.

When I first started getting into horror, the House on Haunted Hill remake was my #1 favourite movie - bar none, hands down. I used to watch it on a daily basis: If I had to count, I would say that I must have seen it at least 50 times - no exaggeration - the most recent being this past Halloween.

In the end, there is a crazy showdown in the attic. Our two surviving heroes - Ali Larter and Taye Diggs - are being run down by a huge ink-like shadow. Just as it looks hopeless, the ghost of Watson Pritchet appears and activates a pulley, opening a huge steel gate and letting our heroes out onto a small ledge. The gate slams shut behind them.

The sun is starting to rise, they both smile and laugh and are full of hope. An envelope is mysteriously pushed towards them from under the door. It contains the prize money that was to be awarded for spending the night in the house. They rejoice for a moment or two...but then the camera pans out, and we realise that they are actually stranded hundreds of feet abouve the ground, and the only way to escape is to somehow pry open the door, and go back through the house...

Trust me - if your description is acurate, this IS your movie. I'm currently at work, but when I get home I'll gladly post some screen caps for you.