Hard to understand a lot's value

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.