Thermostatically controlled gas heaters

I am looking for advice on heating a 15 ft by 10ft polly tunnel mainly for starting young plants

I have a 3 kw thermostaticly  controlled propane heater in my greenhouse and used it for around 16 weeks last winter at a cost of around ninety pounds, for that outlay my greenhouse was at a constant 55 degrees regardless of outside temperature
I did shop round for propane as exchange bottle prices vary by around 8 pounds around here.
Its maybe not cheap, but I get great deal of pleasure from my allotments and thats what makes it worthwhile to me.