you'll pay more for high eff. propane than std. efficiency heating oil. (bear in mind propane has about half the btus (heat) of heating oil so you have to consider that when looking at the price per gallon- that's how the co's fool a lot of people) The heating companies are really pushing propane as it's a bigger money maker for them than oil. propane, on average, vs. heating oil, is more $$ per btu in the past, present, and future. The companies pushing propane over oil don't have your best interests at heart.
Use the excellent fuel cost calculator on this site that was already mentioned.
having said all that, you'll save a considerable amount of money w/ coal even at $300/ton now and into the future. Coal w/ oil backup is the way to go.
What's the problem w/ your current oil boiler btw?