I do like your post. But your numbers and estimates are a little off.
I am in the HVAC business, and what's a little more realistic in the summer is when someone is complaining about high power bills. It's usually in the naiborhood of $350 - $600 a month. Now if A new unit can knock 30% - 50% out of that, that's a little more dramatic than the estimates of $10 or $20 a month. Grant it 50% doesn't happen as often, but it does happen occasionally. Because we now have the technology to speed up and slow down the capacity to better match usage to the actual demand in real time as it changes in addition to higher seer ratings. Therefore only using the energy needed at the time to satisfy the demand.
Unlike the old technology were it was just on and off with no speed adjustment, so it was always all or nothing. For example, That creates waste because you are paying for 3 tons of air when it kicks on even though you might only need half of that on an 80 degree day and then need the full 3 tons on that 97 degree day. They will also save money on repairs, because most new units now come with 10 year all parts warranty. And if done right should deliver much better comfort than that old worn out 15 or 20 year old unit as well.
What we usually see in a situation like that is most people don't realize how bad the unit is under performing until it gets replaced because it has degraded so gradual over time. The rule of thumb with good maintenance being done on a regular basis is a 10% loss of efficiency and performance every 10 years and worse if not regularly maintained twice a year. Combine that with advances in technology, and if done right you can usually create a situation where it will pay for itself much quicker while delivering all the other benefits like better comfort, less breakdowns, property value increases etc. In addition, a lot of houses are not sized correctly. Duct-work not designed and balanced for maximum efficiency etc. All of these things and more have to be considered because they effect the finial outcome of the situation.
Just food for thought. Little more to it than what meets the eye usually. Of course everyone's situations are different, and these are only estimated examples. But everyone has the ability to improve their situation if they get the right company that knows what they are doing and that cares about their clients. We have actually created some situations that put extra money in our clients pockets while the new heating and air system pays for itself. Example, system might save $150 a month off of the utility bill while we got them financed for no money down and only $75 a month therefore allowing them to put the extra money in their pocket every month. Of course this doesn't happen every single time. But similar results can and do.
My two cents worth.