I'm a rotational player with families at many levels of wealth, so I can confirm that the Lot Taxes are only 300 simoleons across properties that range from 13,121 (though with items in sims' inventories) to 467,847. I also started a brand new save in Evergreen Harbor to test things out. The Lot Taxes were 300 simoleons for not only my new sim in his starter but also every single Maxis premade family.
In my (old) rotational save, none of the families have the new power/water generating items yet, so these should not be a factor. Some of them have the "Frugal" reward trait and two families live in tiny homes, and these give appropriate credits in the "Tax Breaks & Penalties" section of the bills breakdown.
I do use mods, but I also tested taking out my mods folder and clearing my cache, which still resulted in 300 simoleon Lot Taxes across the board. (And of course I am only using mods that are updated or confirmed safe.)
I assume this is a bug not only due to the fact that the hover tooltip tells us the lot taxes are affected by item/lot value, but also because it completely negates the main gameplay draw of apartments in the pack that finally gives us new apartments. If my lot taxes are only 300 simoleons on even the most massive and expensive lots, why on earth would I choose to limit my ability to build, garden, and use eco-items while paying a thousand+ simoleons in rent each week (plus deposit and utilities) for the "privilege"? Apartment rent should be cheaper than lot taxes in an equivalent house due to the restrictions placed on them, as it was before this patch. If this change was intended, it was ill considered and should be reassessed.
As a side note, using the thermostat does not seem to have any impact on power usage according to my limited testing. That doesn't seem to be functioning as intended either? It's a shame about these bill issues; I was very excited for the new features when they were announced.