The primary costs can be broken down like this:
1) new tank - 250-330 gallon normal size range, about $1.50-2.00/gal capacity, so about $500-700 tank cost, fuel fill line replacement and filters replacement if needed about $100-150 more. If double-walled tank with leak alarm then about $500-600 more. Indoor tanks fail by corrosion from the inside out from water sitting in the bottom of the tank, so a double-walled tank provides a hollow space between the two tank layers to hold the liquid if it leaks. Alarm sounds if outer tank gets any liquid in it. Another alternative if you figure any leak will start small before getting carried away (the usual case, but not guaranteed) is a composite FRP or concrete catch basin under the tank that can hold 50-100 gallons and has an explosion proof liquid alarm. Of course, unless the alarm is hooked up to a central alarm provider, that does not help if you are away for a number of days when it happens to leak. These measures can help avoid a leak that penetrates under your house slab, which can commonly run $25,000-60,000 to remediate.
2) access - this is the big variable. If you have a hatch, ramp, or oversize door designed to allow easy tank replacement, then $0 additional. If a hole has to be cut in the wall or foundation to allow access, can run $1000-2000 or more.
3) dispose of existing oil and sludge in tank. Occasionally a heating oil distribution company, if they are doing the installation of the new tank for you, will take the oil away for free, to be filtered and used for industrial heating oil or to be sold to a refinery for reprocessing - if so, then $0. They will NOT buy it back from you, because of the contaminants that are likely to be in your tank. Otherwise, if disposed of as waste oil, about $1-2/gallon disposal fee with a minimum of about $100 for the visit, depending on local oil recycling vendor distance. Check with your local waste disposal agency (solid waste services or such name) - some counties also have programs to take unused heating oil from tanks for free
4) dispose of old tank - $200-300
4) installation - about $500-800 on existing cradle or pad, or about $800-1200 including new base/pad, if needed (usually not required on concrete slabs)
5) this one is commonly forgotten about - refill tank - assuming $3.70/gallon, about $900-1200.
One additional thing to consider is the possibility of leasing your tank - many oil distributors will change out you tank for a new one, that is then charged to you on a monthly lease basis, prorated over many years. However, if you are planning on selling the house soon, that presupposes the buyer will be interested in assuming the lease agreement, rather than converting to gas. And of course it will probably tie you to that oil provider for the lease period.