Hiron,
I can help with the pool demo work as we have demoed several over the years, Cities differ in the requirements some require permits some don't.
Most will allow you to cave in the walls but have to bust holes in the bottom so water does not pond during rains.
If you are allowed to cave in the walls this is the least expensive alternative as the concrete will fill the majority of the hole. Thus no transport and disposal fees and less fill dirt needed. I do have a co. I use for dirt hauling and it is typically 275.00 per 12 yards of fill dirt. I then get the last load as good top soil.
The average cost is around 3500.00 -3800.00
If you would like to discuss this more feel free to call me at 682-597-6814
Thank You
Kenny Powell