Its a no brainer... grout is pourous and stone is pourous, if they want to have some semblence of the original color of grout or stone when its cleaned they will have it sealed. And if they don't want it sealed, tell them to color seal or regrout with grey, black or brown grout whatever soil color they have the most of.
You seal to make money. They seal to protect their floors. If the stone and grout requires it, you recommend sealing. If the grout is in a kitchen you seal the grout. If your grout is stained you colour seal. If they are too dumb to realize the benefits of sealing then walk and you will be back to clean again.
Is there an excel spreadsheet that could defined the actual cost savings of sealing?
Like the cost of calling you every 3 or 4 months vs every year or 2.
Like the true cost of floor maintenance with sealing vs without sealing.
In a commercial setting like a restuarant, it is likely the seal is gone or severely degraded by the time of the next cleaning. The products that are used to clean the floor every day are often harsh enough to gradually remove the seal.
A benefit for your cleints is to sell them a product for daily floor cleaning that will not break down the sealer. Some floor care products even contain a little seal to repalce what is lost and keep the seal refreshed.