5e doesn't have rules for this
At least not rules beyond "the DM decides". My personal recommendation is that you price these so you drive the story forwards, which largely means pricing them dependent on their usefulness to adventurers. That's largely how items in 5e are priced anyway.
That said, you get some guidance on the cost of a tower: page 128 of the Dungeon Master's Guide includes the construction cost for a fortified tower; 15,000 gp. If that cost is steep enough for you, you can decide that the cost of 'clockworking' the tower equals the cost of fortifying it. Otherwise, tune that number up or down as desired.
The only priced clockwork object that springs to mind is from the rock gnome's Tinker ability. They can use 1 hour and 10 gp worth of material to make a toy that runs around
screaming making animal noises, what is basically a lighter, or a music box. If you were to buy such an item of a gnome the cost you should include the gnomes time (I don't know what gnomes charge by the hour), and you could use this as a starting point for other clockwork items, but there is little to no guidance on how to do this.