What does it cost to get an ATP certificate from scratch?
The cost from 0 to ATP can be highly variable.  An easier number to quantify is 0 - Commercial/CFI.

Even the cost of 0 - Comm/CFI can vary, but there are a few accelerated programs that offer this.  One service (I'm not advertising, so I'm not naming them) offers a program that lasts 150 days and costs $65,000* + checkride expenses.  You start with nothing and end up with single/multi commercial, an instrument rating and your CFI/MEI/II along with 100 hours multi-engine time.  This particular program (and others like it probably) hires their instructors from within, so there is a possibility for multi-instruction -- but you won't be making much money for it.

The cost of commercial to ATP is harder to nail down because of the possibilities that open up post-commercial.  


On the high end, you could just rent or buy time to ATP mins, but that could cost a fortune.  
On the low end you could work to ATP mins and make some money.  


A good way to make money with a fresh commercial/cfi ticket is to instruct. There are also jobs like banner towing or pipeline/powerline patrol you could do, but you probably still need to build time beyond commercial mins to be competitive for those jobs.  You also mention cropdusting, but keep in mind many of those ops are turbine powered ag-cats and you won't be competitive there with a fresh commercial ticket.  

*You can do it cheaper but I'm giving these numbers because a fast but expensive program may appeal to a career changer.