How do you actually know this? Neither program has even put forward actual cost estimates. The same is also true for specifications and the claim that they will be similar enough to be direct competitors. While this may or may not be true, I don't think we know enough to establish that. Or for that matter whether future administrations will even make F-47 available to export. Sure, Japan may have a set of range requirements..but is that the only attribute that influences performance and/or cost? Was each program offices not interested in any other performance attribute that might introduce variability in performance and cost across multiple different customer requirements on these two programs? We have Japan, US, UK, Italy all very different air forces and user needs but seemingly converging on a very similar set of high end fighter requirements?
The little we know on F-47 is a top speed exceeding Mach 2.0 and a combat radius exceeding 1,000 nautical miles. No idea on size, IWB capacity, supercruise performance,supersonic radius, mission systems, and other capabilities. So there's very little to go by and for GCAP we don't seem to have even that much in terms of similar specs (IIRC).