There is a floor of private consumption of sufficient low quality food to prevent starvation, and minimal clothing and shelter for the environment (the less hostile the environment is the lower this threshold is since clothing and shelter are less necessary and easily available food and water may require little work to produce). This is going to be true for every society no matter how it goes about securing these things. From there, think about long lasting tangible personal property and structures and food reserves.
Another way to think about it is to look at what percentage of the population is engaged in food production. In the poorest society, 100% of the labor force spends all of its time feeding itself with no resources to support anyone doing anything else. (Within those societies, the percentage of the population in the food producing labor force as opposed to caring for and being children and dependents, and the number of calories per person per day can be good proxies for per capita GDP.)
As of 1800, in the United States, 1,405,000 people out of a labor force of 1,900,000 were engaged in direct food production. So, the standard of living that could be supported was about 35% higher than one in which everyone was engaged in direct food production.
Right now, about 2% of the labor force of the United States is engaged in food production, while the other 98% of the labor force is free to do other things, so the standard of living that can be supported is about 5000% higher than one in which everyone is engaged in food production.
The percentage of people engaged in direct food production has declined almost every single decade from 1800 to the present.
This isn't a perfect measure (e.g. it doesn't distinguish between different levels of productivity of non-agricultural workers), but it is a good starting benchmark that can be estimated from information that isn't completely impossible to obtain, and can be used as a staring point. Figure out how many people who rulers, scribes, entertainers, priests, soldiers, craftsmen, bankers, traders, etc. and use those estimates to figure out what percentage of the population was engaged in direct food production. Then convert that to a percentage and multiply by a base number for minimum subsistence.
For much of early history, society was largely Malthusian. Per capita GDP was kept fairly constant and any excess resources were used to increase population until the theoretical floor of private consumption was reached.