I feel like thralls should require a continuous investment throughout their lifetimes. The upkeep would be broken into two parts. The first would consist of what a thrall would need for sustenance. The second would be materials required to maintain the thrall’s clothing and bedding in a manner suitable to their position. I think a basic system could look something like this;
Level I Thralls
-Bread, watery wine/mead/ale.
-Poor cut of meat, wine/mead/ale
-Sturdy cloth, leather.