All through the cold weather, you have to defend the plants in your greenhouse from frost damage and one of the most ignored pieces of gear is a decent greenhouse heater. And the good news is is that you can pick up a decent heater for your greenhouse relatively cheaply.
Now dependent on your requirements, 'cheap' can still be expensive and you can safely assume to spend less or more depending on the size of your greenhouse. If you only have a smallish area to heat inside your greenhouse you can effortlessly pick up a greenhouse heater for the job for less than £30. If you have a bigger greenhouse or need a bit more humph, then the cost of a good greenhouse heater could run into the £100s. As you can see, you need to be pretty sure of what you require before putting your hand in your pocket, else you can end up spending more than what you really need to.
The insulation on most plastic greenhouse have quite a low insulation value and so the buying of the correct greenhouse heater is crucial to replace any lost heat. When extra heat is needed to retain the temperature at the precise levels, you're best off using a paraffin or electrical heater. In addition, a thermostat is vital to ensure you can accurately control the temperature within your greenhouse and fortunately, most variations of heater come with one already installed.
I'd rule out a gas heater as the initial cost in buying one can be a little too high. That's just my view, of course, and I certainly wouldn't advise you not get a gas heater based purely on the price, in fact, they can be a better option in the long term.
The running costs of your greenhouse heater are an essential consideration. The current price per unit of electricity in the UK is about 15p (average) which will see the cost of your running a 1000w greenhouse heater to be rougly that amount per hour. A paraffin greenhouse heater, though, could cost twice as much for the same period of time. You do need, though, to take into account the fact that a paraffin heater manages to preserve it's heat for a period of time when turned off while an electric model is off when its off.
Personally, I go with electrical greenhouse heaters as I favour the upfront low-priced option for the models - I had only a small budget when I started and so the low-priced models and inbuilt thermostats meant I could exactly maintain the temperature with a small start up cost - again, though, that's just my opinion and you shouldn't be put off other options.