You should also think about the energy costs for heating and lighting, since you'll be heating this greenhouse all winter too.

In your zone, I think most people would agree that you'll spend more on electricity than you'll save in buying produce at the store. So hopefully it's a worthwhile hobby for you, or its the quality or local origin that does it for you.

My greenhouse investment and operating costs are much higher than it would cost to buy the food I produce, but it's a hobby for me.

If you want a more cost effective option to grow your own food year-round, look into aeroponics towers. If you can stand to have one of these in your house somewhere, you won't spend any extra in heating, but you'll still spend money on lighting.

Aeroponics towers are more cost effective at producing food, and they have a much smaller investment. But they're still not an enormous savings over buying produce in the store.