It wasn't the price of the plants Call, it was the price of creating suitable environments to plant them into.
For example, a floor level planter big enough to house a 7ft tall Monstera (in time) would have needed to take up approx 50cm cubed of space in the heatsink.
That cube would have to be supported on thick treated wooden stilts driven another foot into the ground at the base of the gravel heatsink then drainage tubes attatched and taken outside the gravel area to stop waterlogging and clogging from overspill.
Then the airducting would need to be routed around it creating staggered joints with shorter runs in betweeen thus reducing the efficiency of the airflow and possibly needing a higher powered fan which in turn would be more costly to run and noisier.
This rerouted airducting would then have the entry and exit points directly in the footfall area, the area where the 'arris end of a plant pot will be sat all winter.
Get it?
And that's nothing compared to the difficulty of planting at the tops of the structure, particularly around the areas where the electrics are routed.