In Mordenkainens Tome of Foes lists Clockworks.  I was wondering if there are any rules for constructing them and the cost to do so.

Thanks Ahead of time

EJM

Will most likely be up to the DM. The only relative rules I know of are listed under the Rock Gnome abilities. It might be possible to scale those up for larger items. 

The best you will find are the Creating an Item downtime rules in XGtE and the Manual of Golems

Manual of Golems gives the following prices for these CRs

Creature, Cost, CR
Flesh       50000       5
Clay         65000       9
Stone      80000      10
Iron       100000      16

Which gives the formula:

Cost (GP) = 27782 + (4596.8 * CR)