IMO it would be flat fee from you to them you need to charge what you feel your time/work is worth
is the work anteriors or posteriors? what tpye of work are they expecting back from you ? are you producing units during your work day or after hrs? is this lab direct competion in your area?
We have done this and charged it out at 70%, they are paying you to make it look aesthtically pleasing and functional. Not an easy balance to strike and way more time consuming than waxing up a substructure.