OK, Lucas drags Mark 60 miles across the desert to collect 2 or so kegs of salt. The trip cost him 2 horses and their gear, a wagon and the kegs of water. On top of that he gets pretty well beaten up. Next time just go to the General Store and buy salt.
I get the point about buying salt but I remember that episode from the original network run as being one of the best of the series. I guess they spent a lot of time and money on the duststorm scene and this episode emphasized Lucas' bond with his son as few others have.
Hate to tell you this, but two kegs of salt at the local general store in those days, if you fast forward to early 21st century dollar values, could have cost well over one hundred dollars. People in those days used to do things like travelling way outside their area to dig these things up, and get the items for free, just as was shown in this episode.
It was one of the best of the series. Lucas giving Mark all his water and food to give the boy at least a chance to live. After Lucas hurts his foot he just can't make it and didn't want to hold Mark back. I do wonder why they don't show Mark eating some of the food in the bag however. There was water in food.