How do sky platforms work?

[WanderleiMMA]

Do you pay once and that's it or is it monthly?

[Rolig Loon]

Two parts of the puzzle:

1. A house is a house, no matter where you put it -- on the ground, in the air, or under the water.  The cost of the house itself is a one-time expense.

2. If you have land, you pay a monthly fee for it, whether you have a house on it somewhere or leave it completely empty. 

So, build a house anywhere you want on your property --- at any height up to 4096m --- and you'll pay the same monthly land fee or rent.

[PeterCanessa Oh]

See also: http://community.secondlife.com/t5/Land/When-you-buy-a-sky-box-do-you-have-to-pay-for-the-area-you-place/qaq-p/2070181