3d games were cheaper than Sierra style adventure games because those style games were moving towards lots of voice acting and full-motion-video. The 7th guest came out around the same time as doom, needed two cd-roms and cost $1 million to make (the biggest video game budget at that time). Phantasmagoria came out two years later, had cost $4.5 million and needed seven discs (8 for the Japanese Saturn release). Doom fit on four floppies and wasn't expensive to make (but I can't find a number).

Of course, today's game budgets are huge --- detailed 3d models are expensive, and FMV cutscenes are nearly mandatory. And games have expanded to 10s of gigabytes.