The true cost difference between assembler/C comes when the project manager/product owner/customer suddenly figures out that they ordered the wrong product, and makes a huge rewrite of the product specifications. It's way easier to move around blocks of C code when "reconfigure" the software to match the changed requirements.

Or the cost of needing to switch processor because it went out-of-life, and the company don't have anyone able to write in assembler anymore becuase the original developer is retired or have moved to another company.

Assembler in itself don't have the name spaces and scoping rules you get in C. So there is no automatic tool that will force locality-of-reference on the code functionality.