I've thought about this a bit, if I were in this situation and this was the only work on the table then in this case I think I would establish a firm set of rules with the person's ordering the work first.
1) Every EICR I carried out I'd want the EIC so I could suitably compair the results, I'd only be happy to sample such a small amount and give it a suitable interval if I could suitably compare results with the original certificate, maximum time being 5 year's.
2) Id specifiy exactly what tests are going to be carried out (which we all agree would be minimal for that price) I'd make it clear in writing first and specify that this must be recorded on the test sheet so no pulling the wool over anyone's eyes, and the inspector is covering himself. (The reason I say this is because I know some company's have admin staff that type these tests up)
3) If no orignal EIC are provided,with satisfactory results I'd have to recommend the date of next test to be carried out the following year as only minimal test were carried out.
If they didn't like this Id explain that test intervals are only for guidance now, and it's down to the profession judgement of the inspector.