Not very common that I know.... usual signing bonus is 20-30K that I have seen (usual is the synonym of average here). This is much better than a few years ago by the way. You can see the average signing bonuses on USNews rankings.... and they are not 100K such as CBS tuition.
Here are the stats from US news for CBS
Signing Bonus: Overall
Total reporting signing bonus 302
Low signing bonus $2,000
Average signing bonus $32,111
High signing bonus $150,000
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