For us this was a 65 mile drive, about an hour each way. It took an estimated 4.5 gallons of gas which cost about $13. We had a picnic lunch there which is basically free (but if I had to put a number to it I'd say it was between $3-5 per person). Throw in the slippers we bought across the street at the gas station and this day out cost less than $30.
On arriving we watched a 20 minute film on Muirs life in the visitor center, this gave the kids an idea who he was. After a quick picnic we headed up to the house and wandered around working on our Junior Ranger books for about an hour, ringing the bell in the tower, looking at his office (the scribble den), talking about the bedrooms, talking about how the kitchen worked, and exploring the attic. We then bumped into a ranger led tour which took us around the bottom floor for another 45 minutes.
After we finished with the house we walked down the hill to see the carriage house, some of the fields and fruit trees, and the Martinez Adobe (a leftover from the Anza exposition). It was pretty interesting to see all the fruit growing and the giant Sequoia John Muir planted across from the carriage house.
I hadn't been to the John Muir House in 30 years, and a friend suggested it recently, so it's been stuck in my head. With summer break upon us, it seemed like an affordable day out.