That seems like an odd versus, right? Well, there's this fellow, Gary W. Clark, who has a YouTube channel, PhotoTree (and a website, PhotoTree.com), and he has a fairly compelling argument for using Orvus Horse Shampoo on some types of gravestone contamination.  His position is sometimes Orvus is all you need and it is much cheaper than D/2. See Gary's video.

In a nutshell, he says there are two types of headstone contaminates:

• First are the biolocical contaminates, such as Lichen, Algae, and Moss, to name a few.  These are living organisms and they attach to the stone by driving their roots into the pores of the stone.  For this, you need a Biocidal cleaner such as D/2 Biological Solution.

• Other contaminates (which he calls 'Black Goo') are a cocktail of tree sap, pollen, dirt, and dust.  This 'goo' collects on the surface and cracks of the stone and for this you need only a detergent to clean it off.  His recommended detergent is Orvus Horse Shampoo, available from Tractor Supply.

Used at full strength, D/2 costs about $44 per gallon (plus shipping).  Orvis costs $40 for 7-1/2 pounds (120 oz.), and you dilute it at 1 oz. of Orvus per quart of water.  At that dilution, Orvus makes 120 quarts of solution, so $44 divided by 120 = about $0.37 cents per quart versus $11 per quart for D/2. (These calculations ignore shipping charges and taxes.)

I think Gary's suggestions merit testing if I were to find a headstone without biological contamination.  If he is right, there is a potential to save a ton of money depending on how many headstones are cleaned.  Of course, if his claims do not hold up, and the Orvis product does not turn out to be useful for headstone cleaning, that's $44 down the drain, literally.