Got Oil ??? In it I mean.....
If it has some oil in it, you may be able to get a local oil delivery guy to take the tank in exchange for the extra fuel.... This only works if there is fresh oil and evidence ( like a delivery receipt of record available ).
Is the tank really a 400 gallon. Most residential tanks are 275 or 330 ( 5 ft tall, about 5 - 6 ft long, and 30" wide ) shaped like a SPAM can.
Here in N.E. we pay about $ 250 to have an empty 275 gallon tank disposed of.
The dry ice idea is a good one. The dry ice evaps to carbon dioxide and displaced oxygen. No matter what, cutting the tank in place will make a mess...the bottom will be covered with sludge and slop that will ooze and stick up the place. Try to get it out before you cut and be ready for some mess.