Briefly, the key issue is to recognize that all data sets reflect and embody the specific theoretical presumptions of those gathering, processing, and disseminating them. Thus, where Marxist categories of thought are excluded, the published statistics reflect that fact. Thus, to estimate rates of surplus value (RSV) requires reconceptualizing and reworking data grounded on theories that were developed (1870s' birth of neo-classical economics) and perfected within a larger theoretical project aimed against Marxism. That is a tall order. There have been a few such efforts usually built around comparisons of per worker productivity (in physical terms) and real wages. However, the enormous difficulties of separating out, from aggregate non-Marxist statistics, the productive workers (those who produce surplus) from unproductive workers (those who receive distributions of the surplus produced by the productive laborers) has rendered these efforts few and largely ineffective theoretically or politically.

Congratulations on grasping that values and prices in Marx, especially Capital, Vol 3, are very complexly related as causes and effects of one another amind the larger overdeterminations that shape them both. Competition within and among industries, differing turnover times and costs, etc can and do make prices diverge from values - that is reasonably well understood in Marxian economist circles - but it is also the case that when input prices diverge from input values, in the production of capitalist commodities, that divergences then enter into the values of such commodities (in short, prices divergent from values also become determinants of values in a never-ending dialectic of price value interactions. None of this should make the analyst, if Marxist, lose sight of the underlying reality that productive workers produce surplus value that the capitalist system distributes, via all its complex mechanisms, to a variety of surplus recipients.

Lastly, be careful in how you use Marx's surplus value theory. Recall that it measures the portion of the productive workers' labor that is devoted to producing such workers' means of consumption vis-a-vis the portion that accrues to employers as surplus value, i.e. the ratio between them. While wages, measured in quantities of use-values that productive laborers consume, are lower in developing countries than in developed ones, that is only part of what you need to know to calculate RSV in both locations. You also need to know the exchange value in those use-values that productive laborers consume. In other words, you need to know how much labor it takes to produce consumer use-values. Thus, as Marx explains in Capital Vol 1, if the consumer goods producers in developed countries are much more productive than their counterparts in developing countries, then the value of labor power in the former may well be less than in the latter countries. I believe that has been the case for most of the last century, and hence I would argue that RSV has been higher in the US than in Africa, for example. African workers live at a much lower standard of wages and living than Americans, but it takes more labor in Africa to produce the wage-bundle of use values there than it takes in the US to produce the larger bundle of use-values in US productive workers' wages. The US working class is thus more exploited than the African. The point here, as Marx made so clear, returns us to the first paragraph above: class and surplus analysis is a different theory leading to different political conclusions than a theory that ignores/abstracts from class and surplus to measure things instead by reference to prices and physical quantities.