You know that Boeing, for one major OEM, charges an arm, leg, and your first born child to make such modifications.
I guess it depends. As a retrofit both big OEMs do that, however, boeing offers it basically for free on its 737NG, 747-8 and 787 types. Probably will as well on the coming reengined versions of the 737 and 777.
We have GBAS capability since 2006 on our boeings (and those have RNP 0.1 as well) and when the decision was made to phase out the boeings in favour of airbii one of the questions was how much it would cost to install GBAS on them. But with around 250k per airframe for a narrowbody airbus there simply wasn't a business case. However, we have to start getting RNP AR approval now, previously we could simply boeings only on those limiting flights.