$750-$1000 for a full rebuild. Probably not worth it if the pump is otherwise working fine. This entails full disassembly of the pump, inspection of everything, replacement of any worn parts (plus replacement of all gaskets/seals), and calibration to Bosch specs.
Under $100 to re-seal it yourself. BT, DT. The bottom O-ring is usually the worst offender. Also change the side gasket, rear governor plate gasket, ELR O-ring, vac shutoff pod O-ring, block/crankcase O-ring, and the shutoff lever shaft O-ring. The only seal you cannot replace is the L7 rack sensor O-ring, this is internal and requires full pump disassembly to replace (not DIY at all). If you are only having problems with the side plate & shutoff lever, why not buy a new side plate (assuming the original is warped), and change the shutoff lever O-ring?
A third option is to find a diesel shop willing to re-seal the pump for you, and put it on the test bench for a calibration as-is, without doing a rebuild. If you can find a place willing to do this, figure $100-$250 labor, plus gaskets. Many don't want to bother and instead try to sell you a rebuild job.
