My RTGG cost me £20 when he was 3" long. He's now a very thick 17" so you'd think he must be worth several hundred, even nudging a thousand pounds.
If truth be told i'd struggle to give it away, let alone get any money for it. LFS's don't want to know because they don't always have the tank space for such big fish plus they'd struggle to sell it anyway, and in the meantime they'd have to feed the monster, at their own cost.
Giant gouramis just don't seem to be a very sought after fish when they get big. It makes sense really. They grow at a rapid rate so it makes more sense to buy a small one and grow it out.
On the other hand, as an example, big clown loach are like rocking horse shat and you can get silly money for them all day long.