Many (most?) of the premium quality capacitors and inductors used for high-end passive crossovers are larger (sometimes significantly) than the more economical parts often used for base-level speakers. I don't know which parts Spatial is using, but I expect upgraded parts that are enough better than the stock parts to make a worthwhile upgrade are not going to fit in the same space, and will require an external box.
So in addition to the electronic components themselves, the vendor has to supply a box and additional cables and connectors, and these have to be high enough quality to not detract from the base speaker.
Just as an example, Alsyvox offers external crossover upgrades for some of their line. Their speakers are dipole planar-magnetic speakers similar in concept to Magnepans, but a couple steps up in quality (both sound quality and finish) and price. The external crossover upgrade for their Botticelli model retails for $29,400. You might think this is ludicrous, but when you add up the cost of the components, high quality box, connectors, cables, assembly, shipping, etc., it is not an unrealistic markup.
For a more modestly priced speaker system like the Spatials, the "upgrade" parts probably do not have to be as high-end in order to make a difference, but the price ratio (Alsyvox's upgrade crossover is approximately 30% of the base speaker price) is not at all unrealistic and may even be on the low side for speakers like the Spatials (which offer excellent performance for the price already).