Codeshares, standalone, often are more expensive, as a general statement unrelated to BA specifically, other airlines do the same. You are getting some things you won't get on an EI prime booking (TPs, Avios, Eligible Flights, lounge access in certain situations). Also when linked into a BA mainline booking they can be cheaper as well as giving flight connection protection. For corporate customers - whose role is often understated on this forum - the money spent on codeshares can role into any annualised deal.