Insulate the base, back and sides of the tanks with 1 inch thick polystyrene foam sheets.
Have a coverglass on top of each tank to stop heat loss. Use glass that is 4-6mm thick because it is less likely to chip and crack compared to thinner glass (say 2-3mm).
If you have a window, make sure it is double glazed or put a 4-6 inch thick block of polystyrene foam in the window when the sun goes down. This will stop heat loss overnight.
Seal up the room to trap heat and the power bills shouldn't be too bad. But aquarium heaters do use power so the better insulated the tanks and room, the less power used.
Plants should have around 12 hours of light per day. If they get less than 8 hours of light per day, they won't be growing well. Your lights will also help warm the room so a well insulated room should warm up during the day when the tank lights are on.