As life-time value is a prediction of the revenue attributed to the members on a site, it is difficult to provide a definitive value for this as it will fluctuate per member.
To calculate a site's LTV we would recommend using your campaign tracking data. If you have many campaigns running with large enough data sets it would also be useful to calculate LTV at a campaign level so you can optimise them accordingly.
Calculating LTV using campaign tracking data
Your campaign tracking data can be found in the Partner Portal for each of your sites. The Partner Team can also provide a more comprehensive version of this for you at your request to include all sites combined.
We would recommend putting aside the most recent 6 months of data as these members won't have had a long enough re-bill period to draw conclusions from.