Except that we do. We assign monetary values to human lives in actuarial models every day. The statistical value of a human live in the Netherlands when considering traffic safety was E700k in the last project I worked on, but this varies by purpose and over time. Furthermore, there are several international standard methods of quantifying the burden of disease, like the DALY and QALY.
So yes, we can and we do 'objectively' (as in, per a standardized methodology) assign value to human suffering.
Yes, by using a model. The assumptions of the model are subjective in the sense of making judgments about what's important. For example, they don't take into account catastrophic collapse of the environment (overfishing, species extinction, ecological collapse eg from bees dying out, acidification of oceans from CO2 release). They also make assumptions about people's utility functions -- the famous example by Bill Gates that baldness has much more more research being done on it than malaria relative to the suffering, because people with more money speak with a larger economic voice and skew the results.
In your model, given certain starting conditions, can you ever get a positive value for human suffering? For example if rich people are willing to pay to see gladiators fight animals to the death, or just to see the poor people subjugated as slaves. I am pretty sure you assign positive financial value to animal suffering because factory farms torture animals to save a buck and therefore in purely economic terms they are doing "better".
So yes, we can and we do 'objectively' (as in, per a standardized methodology) assign value to human suffering.