I would say that on average I will protect about 1 in 4 of the suites I clean and generally charge £65 - £75 for it. Protection on carpets is a lower hit rate, and I generally charge the same again as it cost to clean it (£45 to clean, therfore £45 to protect, makes life easy!)
For demonstration I always have a sample of material and carpet in the van, half of it has been protected and half is normal. Demonstrate by getting some water and splash it all over, hey presto the water soaks straight into the unprotected half whilst is just runs off the other half!! WOW they all say!!
I always use Extracta Stainguard XP solvent based protector, they do supply a water based one which dilutes down to 1:5, but I prefer the solvent one as it dries quicker and performs better, costs £33.60+vat for 5 ltrs.
With a good double pass application using a power spraver and fine nozzle you should get at least two suites done from a container. Always tell the customer that it is not 100% protection, but is designed to give them time to go to the kitchen to get a towel and blot up any spills before they penetrate the material.