It could be as cheap as a $25,000 remediation , and go into the $100,000 range it all depends on the site conditions and how far the oil has spread . If the oil tank is close to the dwelling , oil could be under the slab . It could be as small as a 275 gallon tank all the way up to a 1500 gallon tank . Back in the day people would install the large tanks and got a great price on a fill up
Find out how much it would cost to remove it and go from there. If it were me, I'd get the current owner to have it dealt with before I took possession.