First, i would would find out what your city;town ordinances are/requires. Here in MI usa my husband did a pool fill in and it was required that everything had to be gone and inspected before filling in according to one city's ordinanaces;plumbing,walls and all; another city just concrete [at 6 ft depth or deeper] could be left with clean fill over it. The bottom does have to be drilled for drainage; I would guess the metal{wall materials} would have to go ,as well as the liner, minimally.
He has charged about 3,000$ dollars including bringing the dirt in w/ a small loader. hope this helps.