The carts, and replacement parts/repair kits/rain covers/etc, along with other public witnessing equipment, are provided to congregations at no cost as part of the standard literature order/delivery, at least here in the UK.
I do believe that there was a 'suggested donation', which AFAIK was a relatively reasonable amount, when the Trollies where introduced, but this appears to have been dropped now.
BUT
it's probably a bit like your computer printer... (cost of actual printer compared to ink cartridges)
The CAPITAL cost of the Carts almost pales into insignificance with the Carts RUNNING costs.
Running costs? Yeah! The large scale (often sticky-back) posters that need printing
https://publicwitnessingsigns.com
http://esignbuilder.com/jwimprintsstore/
That's all folks!
DarK SpilveR