May I ask how much more the basekit would have cost with those 3 keys added? Working on getting them included with more base and extension kits from different designers and coming armed with that knowledge would be helpful.
Its not so much that the cost is a ton, but it is up to the designer to choose combability to layouts that they think will be profitable (or that they like to use). There's a few other layouts that fall into this category: 3 key 40s, split backspace, 2u shift, r3/r4 pg up and pg dn. Hell, even NumPad. Supporting ALL these layouts is expensive, so GMK designers need to follow their own path some and support the layouts that they think will be most successful. More recently, 3 key 40s has been raising in popularity while HHKB has gone down. Another solution some designers will follow will be to cut back on accents.
Physical HHKB is particularly tough because a lot of HHKBers are fine without physical support. Or will buy a Nov kit.