That sounds high for just the balljoint on one side.
If you get a whole new complete arm in the deal, ok.
This procedure is not rocket science, so you might be able to find a better deal.
The only potential 'gotcha' is that if they replace the control arm bushings, the bushing orientation is important, and a non-MB mechanic is unlikely to be aware of this.
1986 300E 5-Speed 240k mi.