I usually don't bother with inspections as beaver are beaver generally. Don't want to waste my time or a clients money. It helps to ask as many questions upfront, is it a natural pond, a stream, river, man made pond, where it is located in relation to people etc. If there is something weird to deal with you can explain it and charge for it. I do a set up fee and a per beaver charge most of the time, but of course there are many ways it can work. I have some that are per trip minimum. A lot has to do with the area, what the competition is charging.
With that said, there has to be something figured in somewhere for a per trip fee if beaver are wised up from what you inherit from other trappers. Some beaver just do not respond, but avoid from day one, from nothing you did incorrectly, can be maddening. A colony a few miles from home may take a little extra time, but a job 40 miles away can be a big loser. Can't say every job over the years has gone well for sure.
If a job does goes as it should with an evening set up with a new pair of beaver, the job will be done in 12 hours with both beaver caught the first night, easy to price and a money maker. Always helps to have multiple locations close to each other if possible so that there is profit from the good ones to absorb some extra time with a bad one. Your mention of one beaver is important for two reasons, obviously only one beaver to get payed for if you are doing a per beaver charge. Almost have to have a two beaver minimum figured in somewhere, or the per trip fee. One beaver takes more time and work than two most of the time. If you catch a matched pair, M and F, it's easy, done. If you catch one, you never know for sure if there is a second, so it requires another trip at least, sometimes two or three depending on how educated the second is.