This question is quite open to interpretation in terms of what actually counts as "the coaster". I heard the 100 million figure being bandied about quite a bit during the Singapore Revenge of the Mummy construction. However, rcdb and Wiki both report $20 million. Now even if the 100 figure is Singapore dollars, that still equates to about 80 million US$ and to be honest I can believe that's right.. for the WHOLE attraction. I'm sure the $100 million we hear for Expedition Everest wasn't just Vekoma's fee. It probably covers the entire investment. We think of Mummy as a Premier coaster because we are enthusiasts but to most people in the industry, Revenge of the Mummy is an ITEC ride. Premier are just one of the vendors, like the lighting guys and the ventilation people... they just happen to supply the coaster. You have to say that the rcdb figures are a bit random when they quote the Hollywood Mummy as being twice the price of Singapore. Hollywood already had their building, where-as Singapore built an entirely new facility, added some serious theming to the front (another building in fact) and the show equipment, flame effects and shear volume of the scenic spaces are far greater than Hollywood's (from memory). That's before you consider inflation.
So, yeah, if we are talking total investment, then Mummy x 3 will be in your top 10.
After that I'd guess Manta, the new Disneyland Space Mountain (no idea what they did in there but Dynamic Attractions would have had a big fee and Disney are sure to have spent big elsewhere), probably Formula Rossa. Leviathan won't come cheap. California Screamin, I'm pure guessing now.