I'm from the UK so I fully understand your cost issues. Our energy prices have gone daft over the past couple of years. We have a wood burner which heats the house remarkably well. I still have heaters but the cost is manageable, it's not as if they're on 24/7 because of chilly room temps.
In the UK I'd say a conservatory is probably the worse place you can put a tank. You think you have problems in winter, wait until summer comes, especially if your conservatory is South facing. You'll be forever trying to cool your tank down!
As far as asking, "how much is it actually going to cost me?" No one can put a figure on that, there are too many variables to consider. The best thing you can do, great if you have a smart meter too, is to monitor your electric usage over a period of time before you actually set your tank up. Then monitor your usage for a period of time whilst the tank is running.
The average difference in cost will be noticeable yes. Not only will your heater be pulling juice, but any lights, filters, powerheads etc etc, will also be adding to the cost.
Once you have the figures you'll know exactly what that tank is costing you and you and your partner can then make a decision whether it's financially viable for you or not to run that tank.
Yes, you can insulate your tank, which will help. But how far do you take that before your display feature of an aquarium turns into an eyesore?
And yes, you could also embrace the coldwater aspect of the hobby, although even without heaters there will still be cost from "other" gadgets.
Lots to think about, I wish you luck!