The only other mount changing service I know of is Leitax.
They charge anywhere from 58 to 75€. for the mount hardware and another 50€, if you want them to make the swap plus shipping. So, that is about 130€. for the entire process. Leitax had to do all the reverse engineering and parts manufacturing. Additionally, they support Contx, Leica, Olympus, Voigtlander to a wide range of mounts, K, Nikon, Canon, 4:3, M42, etc.
This has to be a lot simpler for Sigma since these are their own lenses and designs. No reverse engineering, plus they have all the original parts, and knowledge of the target mounts. I am not against anyone making a profit, it just seems that the freight is a bit high.