LPG is probably the most expensive form of fuel on the market, not counting "brown" fuels such as coal, peat, etc. Gas prices usually follow oil prices. Oil peaks as does gas. In my opinion, as it has done so before, will drop again and then rise to a peak again, so we panic when it does and breathe a sigh of relief when it comes back down again. Natural gas is the cheapest form of fossil fuel energy, then oil, then electricity, then LPG. Before you decide, make sure you talk to people who have it installed and see what they say.
Before going to the expense of changing, try make your home more efficient, improve your heat losses and improve your heating system. Grants for many improvements are still available, (I hope!!!).