Maybe Merck paid Ariad $70 million, not for sarcoma, but because their IGF1R inhibitor won't work well unless combined with an mTOR inhibitor.
Fine by me.
IIRC, IGF1R is a significant factor in prostate and breast cancer -- a far bigger market than just sarcoma. Whatever they plan to use it for, it means FDA approval (= milestone $) and sales (= 10+% royalty $).