The 3D models used (character, items, locations) are made by professionals. Anyone can then buy the right to use the assets in games they make. So if you buy a lot of assets for your game, it will add up quickly. If the game goes on for a long tie and you want to keep using new stuff to keep it fresh, it will also add costs. DPC uses a lot of new assets each episode.
If you play enough games that use
You must be registered to see the links
and
You must be registered to see the links
(both stores for 3D assets) you will start to recognize characters and places from other games. This is because multiple developers are buying the same assets to use (obviously the highest rated assets on the stores will be used a lot).