I typically leave a bedroom and a kitchen window open a crack, for ventilation. This time of year this lets in cold outside air that has to be heated to keep the flat warm. This costs money, but we can estimate that cost. See if it is large or small.
Estimating how much energy you need to heat air is easy. The heat capacity of air is roughly 1 kJ/K/m3, i.e., you need 1 kJ (= 1000 Joules) to heat 1 cubic metre of air by 1 K (= 1 C). My bedroom has a volume of about 35 m3, so if the air that comes in is 10 K (or C) colder than inside (eg if it is 10 C outside and 20 C inside), then heating it up takes about 350 kJ*. For gas the energy cost is about 10 p/kWh – the standard unit of energy used is kWh – the kiloWatt hour, which is 3600 kJ. So in pence per kiloJoule, energy from gas costs about 1 p per 360 kJ. Heating the air in my bedroom by 10 K costs about 1 p.
So, if ventilation turns over air about once per hour then that should add about 1 p per hour per room to heating bills. I’m happy to pay it, and can pay it, but I do worry that people struggling to pay energy bills are under pressure to save every penny, even at a cost to the quality of the indoor air they breathe.
* This for air coming through a window. A very energy efficient home should have ventilation with heat exchangers, i.e., some sort of (possibly mechanical) ventilation that uses the outgoing warm air to heat the incoming colder air, and so keep the air inside both warm and fresh, at minimal cost.