Just to compare apples to apples, Kim San's price includes a SUV. The $35 for a guide isn't inclusive of that - at least, not the last time I checked.

Not sticking up for Kim San as it was my colleague that got fobbed off to another guide but there the prices quoted aren't always similar.

Whether someone thinks a driver is worth $20/day is another discussion. I've seen people doing it in tuk-tuk but the afternoon heat is pretty unbearable. (A bicycle will be almost lunacy.) Tuk tuks are fine for the 2 min ride into town, but that's about it.