Every first-round interview is already costing you something.
In-house, it is paid in senior engineering hours. At an agency, it is paid per interview to someone external. Either way the first rounds are the expensive ones, and they are the ones least likely to change the decision.
Where the cost actually goes
The first round is the least decisive
Most first-round interviews end in a no that an earlier, cheaper signal would have reached — and they occupy the people whose time is hardest to replace.
Agencies pay it as a line item
An agency buying freelance interviewer hours per submittal sees the cost directly, which makes the arithmetic easier and the conclusion the same.
What Narad changes about it
The expensive round moves later
Resume, voice and technical rounds run without occupying anyone on your side, so the human interview is the first time a person spends an hour.
Pricing follows your volume
There is no fixed shelf price, so what this is worth depends on how many of those hours you are currently buying.