The cost has always been the sum of:

1. The time spent to think and iteratively understand what you want to build 2. The time spent to spell out how you want to build it

The cost for #2 is nearly zero now. The cost for #1 too is slashed substantially because instead of thinking in abstract terms or writing tests you can build a version of the thing and then ground your reasoning in that implementation and iterate until you attain the right functionality.

However, once that thing is complex enough you still need to burn time on identifying the boundaries of the various components and their interplay. There is no gain from building "a browser" and then iterating on the whole thing until it becomes "the browser". You'll be up against combinatorial complexity. You can perhaps deal with that complexity if you have a way to validate every tiny detail, which some are doing very well in porting software for example.