I've looked for something similar to you for my van but haven't found anything yet, the Sunngas parabolic heater seemed to be the best option but at the moment I am still using a camping gas cooker with butane cartridges, and though probably more expensive to run, it's perhaps not as bad as you think and it makes me be more economical and thrifty about putting it on and I put a wool jumper on instead and only switch the cooker on when desperate.
You can get the butane cartridges for round about £1 if you buy in bulk on ebay - I've just bought £28 at £1 each. In the summer they are easier to find - B&M Bargains and Poundstretcher do 4 for £3.99 but don't stock them over the winter.
I estimate that each canister lasts about 3 hours, longer if turned down low, so that will give you some sort of comparison if you know how many hours a butane gas bottle will last.
I use one of these on the gas cooker and can definitely recommend it, it sends the heat out sideways instead of just rising straight up and it makes it safer, so that the naked flame of the cooking ring is not exposed to a dangling sleeve etc
http://www.amazon.co.uk/Bright-Spar...74&sr=8-74&keywords=Heater+butane+camping+Gas
Just make sure that when it's really cold, you take the butane gas cartridge to bed with you at night, as it needs to be kept above 5C to work properly. That's where the cartridges have the advantage over the bigger bottles!
Keep us posted in what you eventually settle for, I'm still pondering getting the sunn gas heater and may do if the weather turns very cold but at the moment I'm managing with the butane cooker and also a bio ethanol heater (a stainless steel sugar bowl from charity shop inside a small cast iron chimenea which you would usually have a candle in but which acts like a radiator and remains warm long after the fuel has burned out).