A digger in many rural areas costs around €25-30 per hour, but he'll need to be paid to bring it there too (and to get home). So €100 doesn't seem outrageous unless the digger is onsite anyway. Are there other jobs you can get him to do when he's there?
The hole has to be left awhile to fill up so the guy has to go away (with his digger) and come back, so you pay all over again, naturally (unless the digger is on the site doing a few days work clearing your site, digging foundations, drains etc.). But you're probably not ready to clear the site if your PP is depedent on your percolation test, I guess?