@Fie
Acumatica's out of the box Rule Conditions ONLY can compare a selected field to static values. Both side of the equation can not be dynamic.
I submitted this request two years ago to be able to compare fields of an entity with each other or with other entities or even better have formulas but I have not seen it’s implemented or any possible time line.
your case is not very complicated compared to what I was going to achieve at the time. You easily can add an unbounded calculated field to the line DAC Ext and subtract Cost from Price. This will be a single field that you can use as criteria in your condition and check if is less than zero.
This is the only available option. Unfortunately without this small customization you won’t be able to get your desired result.
I have worked extensively on Acumatica’s Approval Map and have developed a pretty robust customization that can be used by other customers as well. No strings attached but if your needs are more complicated that requires more flexibility I can give you a demo.
Acumatica’s approval is pretty good for straight forward approvals but you will see when become more familiar on rejections and writing comments on Approve/Reject you will face shortcomings.