I'm actually in the process of getting a pool filled/removed at a property.
Basically two options (this is the info I've received, hope it's correct).
1. Fill it in, only need to crack the bottom for drainage and fill it up. Depending on access anywhere from $5-$8k. I was told you cannot build on top of this.
2. Demolish it. You can generally build on top of this if it's been compacted really well. Quoted anywhere from $9-$13k.
The one I've got is on a main road with very tight access, which means it's more expensive and also means the heavy trucks need to use the driveway (which is another possible expense to repair).