Hard to understand a lot's value

I would say the correct lot value is the sale value. Furnishings on a lot always decrease in value over time, and even items received for free can be sold for sometimes high amounts of money. In Buy/Build mode the catalog price is one value, but as soon as you start play mode and then try to sell the item the value is much lower than the catalog value. Not sure if the value increases or is affected in some way if you do upgrades on the furniture.

If you want to know the value of a lot sans any cash on hand if it's owned and minus the initial lot price, you need to look at the value as if you were going to upload it to the Gallery. In build mode from either Manage Worlds or while loaded into an owned lot, hit the 'save to library' button at the top of your screen and then 'save lot'. You don't need to actually save it, just look at the price there, that's the true value.

I've basically just expanded a little on what @crocobaura has already shared. Also, doing an upgrade on objects does not increase their value. In fact, upgraded objects when uploaded to the Gallery or saved to your library lose their upgraded state. Hope this is helpful! :)

EDIT: I don't believe anything in your families personal or household inventories is considered in the true lot value. I have no idea about any money in the GF safes. I have a feeling though that isn't a consideration either. But someone else would need to clarify that as I've not used one of those yet.

Thanks @crocobaura @Karababy52 . I often need to know the value because "calculating local taxes" or doing heritage. In my game (rotation) the main heir will need to pay out siblings with their fair share, but not knowing what s/he actually inherits makes this difficult. I wish the game could have a clearer wording re what their given values include, and that there would not be a different value depending on what mode you check in. Shouldn't really be needed to ever wonder what a given value includes. Like in my example, nothing seems to match from different modes, and I'm forced to guess the right value.

@Simmerville if you are doing inheritances, MCCC has the ability to automatically distribute inheritances to offspring when the parents/grandparents die and you can set the percentages there. It takes a bit to get used to the menu, but it works really well.

In case you don't know, MCCC is a popular mod that helps with things you'd normally have to cheat or do yourself in a roundabout way. Deaderpool is the creator and you can download the mod from his patreon.