It's great that you've already insulated your greenhouse with bubble wrap and have plans to use thermal masses. These steps can help a lot. To make it even more cost-effective, you could think about these things:
Use a thermostat: Consider getting a thermostat for your heater. This can help keep a steady temperature and prevent the heater from running too much.
Check for drafts: Make sure there are no gaps or drafts that could let the heat escape.
