I Think that sounds about right, $100 from 1934 inflation adjusted comes out as about $1,500 today, it sounds like they were doing a fair amount of scrounging to get there and assuming free labor. If you assume about a 50% savings thanks to the scrounging factor, that will bring the budget up to around $2,250. The example pool appears to be 28 ft long and about 12 feet wide, with a 6'6" deep end and about a 3'6" ft shallow end (to the rim) being built as partly above and partly below ground. I can see this being within the realm of physical work a pair of men in fair condition could do in their spare time over the course of a few weeks. Consider a modern above ground 33x18 foot oblong 52 inch deep above ground pool complete with filter and pump can by bought from Sam's club for under $2,000 and set up for far less labor. As to the labor my 20x40 foot pool was dug by hand 30 years ago, more importantly it was dug by hand by a crew of teen age boys in 2 days, plus another 2 days to form up the sides and add the liner, plumbing, set in the ladders and diving board, and pour the deck. From marking ground to swimming in the course of under a week, with the oldest person on site for most of the construction a 20 year old college student (who happened to also be a neighbor) (the adult contractor supervisor was also supervising the construction of 3 other pools that week so would drive back and forth between them as issues came up). These were the same boys that would work picking water melons, or stacking hay, they would often be high school football players who wanted to stay in shape over the summer break and make a few extra dollars. These days these type same kids requires summer camps with professional exercise equipment to stay in shape, I can tell you from experience picking 30-40 pound water melons and loading them on a trailer being pulled across a field at a fixed speed behind a tractor can provide anyone with enough exercise. You get all the right incentives, you get paid per melon picked, pay for the ones you break, you get to run across a field with obstacles/tripping hazards (vines, snakes, etc) you have performance goal set by the guy driving the tractor, if you don't keep up you don't get paid. You learn team work, pickers toss to loaders, and they rotate jobs to stay fresh. Oh and of course the best part is there is always a chance to eat water melon at the end of the day, because some always break.
Ike