There are some ways to try and make the heating that you do have a little cheaper, like load the greenhouse with things called "thermal mass." Thermal mass is material that soaks up heat energy, without actually changing temperature to much, these are beneficial because it holds and sucks heat during the day, and at night, when the thermal mass is much warmer and has more heat energy than the surrounding air, it starts to release that heat. Thermal mass consists of things like, gallons of water, Cinder blocks, bricks, and some large dense plastics.
Another creative way to help heat the green house is by simply digging a pit were the greenhouse is (this can be an anuall thing if your GH is mobile) and lowereing the whole greenhouse into the pit. It should help in getting more consistent temperature because the ground is so well insulated the geothermic heat is going to help somewhat.
The way my dad helps heat the chicken coop, is he built this interesting device, all it is, is a large box, with a window, so it looks kind of like a screen door, plus a door right behind it that you can see. The inside is painted black. It heats up in the sun, you could use a pressure system with water, but my dad just uses a small fan, facing inward to the chicken coop (or you could use a toob) The fan is solar powered, so when the sun is out the fan starts up and the heated air that moves through channels is directed into there. The air, even though it probably has somewhere around only 5-10 minutes to heat up as is moves through the channels it gets to about 80-90 degrees in that time.
Another way, sometimes improbable but my neighbor who owns a greenhouse does this. If you have a spring or natural run off in your location (for instance my neighbor has a spring that goes through his yard) He runs the water accross a channel inside the green house, since the water is almost always 42-50 degrees, it helps a lot with heat, plus added humidity helps keep the heat in.
So anything that you can do to help retain heat, or keep passive heat will in the end make a difference, still however there is no practical and cheap way to heat a greenhouse, not if you want the temperatures higher than 50-60 degrees, especially at night
Hope this helps