Like other have mentioned below, you will want to get estimates completed by reputable companies.
This is very important.
Good luck to you!
You need to be CAREFUL and go with a reputable company. I have also seen a lot of scam companies that charge $500 testing fees, and come in thou none of it was thousands above estimates. I have seen places tested 2 times.. with NO problem findings and then a third company called in that said the site needed a $17000 clean-up.
Be sure you get in a qualified company.
You are at their mercy and they can scam the daylights out of you and the seller!
The cost of the oil tank removal will depend on many factors among them are accessibility and size, but that is only the tip of the iceberg. If the soil is contaminated, the cost will skyrocket. You will also need to have a soil test done, so the total cost will be more than just tank removal.
For complete costs in your area you should contact at least 2 companies and get estimates. Your Realtor should be able to help you find an choose a tank removal company.