Suggestions for model fit with unusual data set?

Good morning,
I'm struggling to fit a satisfactory model to a data set I have resulting from a systematic review. I performed a systematic review of estimated $ valuations of grasslands and the services they provide. All grasslands provide all services, but some studies only value one of these, and others value the grassland as a whole. I want to perform an analysis which helps to explain the variance between values, but don't know how to structure in the component parts. Lets say a grassland value is a function of:

Total economic value (TEV) = Service 1 + Service 2 + Service 3 + Service 4
n=20 n=10 n=15 n=3 n=17
where n is number of studies I have found valuing this.

None of the services, or total values I have obtained have more than 20 studies valuing it, and I cannot seem to do an ordinary meta-analysis as the original studies have no effect sizes I can think of (some are valued via surveys to the public, some are estimated from current carbon costs etc.)

If anyone can suggest a model which can take in to account component parts of a whole, or can handle small sample size (I would be happy to analyse only the TEV papers), I would be most grateful. Please let me know if you need any clarification.