"The key is to find detailed information about the survey design and figure out what is the population surveyed and how large is exactly the area studied. Then you can calculate the total value by multiplying the per person (or per household) value and the number of people (or household). The next step is to divide the total value by the size of the study area.
The steps are easily said than done, of course. Mostly because valuation studies are not designed for benefit transfer purpose, as a result, one cannot find enough information about survey designs. This is especially the case for journal articles where authors tend to focus on the innovative side of the research and use very little ink on the survey itself. Then you can either contact the author or track down other publications that were derived from the same survey. A project report, for instance, often carries more detailed information about the survey.
If you still cannot find enough after trying these two approaches, the last resort is to make you own assumptions. One has to be extremely careful here, because this step might introduce errors. There are two things you can do to make yourself feel less guilty. First, to err on the safe side. It is generally more desirable to come up with a conservative value rather than an outrageously big one. Second, be transparent by keeping a very good note about what exactly you did."