i think it’s that expensive simply because the developer chose to price it as such. (probably more to it than that as they know how much time/knowledge they put into it and have placed a monetary value on their effort… their mindset may be more of a professional programmer as opposed to more of a hobby mindset?)
i suppose it offers unique capabilities which could also be a factor in its price.
that said, there are a lot of free or cheap extensions which are no simple feat and require complex algorithms etc in order to function…
i suppose a high price point could (should) also be accompanied by professional support so maybe tgi3d offers such with their software? (not sure… i’ve never used it)
really, it’s just down to what the writer wants to charge and/or how much they think it’s worth or how much they think people are willing to pay.
outside of sketchup world, other 3D modeling software which has plugin abilities – you’ll generally see much higher price tags for a plugin than you’ll see with sketchup… in many situations, the plugin will cost more than the software.