Single Skin Oil Tanks start at around €215 + VAT. (You will also require a tank kit, i.e. fittings, isolation valve, remote acting fire valve, etc.)
Bunded Oil Tanks are very expensive and start at around €650 - €1,200 + VAT. The more expensive ones may have more features, such as built in level indicators, overfill alarms, better secondary containment, etc.
Either way, make sure either oil tank has an OFCERT. (Certified by OFTEC) (Oil Fired Technicians Assocation)
Note: Ensure that you only require a single skin oil tank, i.e. your oil tank is located more than 10m from a gulley, 50m from a bore hole, well, waterway, water table, etc. and is not on hard ground that is sloped to a public highway, etc.
Otherwise, the oil tank MUST be bunded. If you fit a Single Skin Oil Tank within the above situations, then you should check with your insurance company to see if you are insured, should there be a leak, etc.
One case I am aware of is: oil tank split 2 days after fill, 1,000 litres of oil into waterway, client claimed from insurance, insurance company by law informed EPA (Environmental Protection Agency), EPA submitted bill for clean up for €300,000, insurance company refused to pay as oil tank should have been bunded, homeowner liable for bill, ongoing, etc., etc......
Moral of story: Check with your insurers. Over the lifetime of the oil tank, many 10's of thousands of euros of oil will pass through it, for the extra cost, put in a bunded tank.
It's safer!