This is a heat transfer problem
You can get a first order estimate by considering only conductive heat transfer, neglecting radiative and convective. Simplifying it even more, (again we don't know anything about your building, so this is the best we can do)
q = k*dT
where q is the RATE of heat transfer, dT is the temperature difference between outside and inside, and k is a constant that we determine experimentally.
Let's assume it's 10 degrees C out. The average difference between 20 and 10 is 5 (that is, dT changes every time the temperature changes, but we're simplifying, so let's set it to somewhere in the middle). So we'll let dT = 5. So, per hour (or minute, or second) you have 5*k units of heat exiting the building.
Versus 8.5*k units of heat per hour if the temperature is set to 27 C.
So, how much do you pay per unit of heat? Multiply that by 3.5, and there's the savings
Keep in mind this is a VERY rough estimate. I've neglected a LOT of factors. Your problem may be a matter of configuring the HVAC system for optimal air flow, or getting new insulation. I'm neglecting the shape of your building vs how the wind/sun impact it all day/season/year long.