You may want to check other parts of the heating equation if you think your expenses are too high. Insulation, boiler maintenance, boiler size(may be able to downfire an oversized boiler).

If your boiler is at the end of its life it could be worth switching but if you just switch because "oil costs" you will take a very long time to break even and if you have insulation issues it may end up costing near the same in gas per month.