score:28
I couldn't find one from 1940, but here's a snippet from a 1950 ad for a "Koroseal garden hose"(commercial site link). They were offering a 75-foot hose for $13.20, which is certainly in the same ballpark as FDR's "$15" a decade earlier. You could save a few bucks with a shorter hose, but if you're trying to put out a housefire, one would imagine the longer the better.
You might think it likely it was a bit cheaper 10 years prior, but it appears that hoses at that time were usually made of rubber, and with WWII in full swing in Europe, it seems likely the demand for rubber (and thus its price) was at a relatively high level.
Stuart F down in the comments found an agricultural supply house selling (shorter) 50' hoses in 1940 for $4.95 tops, and Andrew T. found an ad in the Morristown Daily in 1943 offering them for $3.33. So it seems like under $5 would have been a more reasonable price at the time, and perhaps under $8 for a 75'.
Of course its also possible that part of the context of the analogy was that the neighbor was reluctant to part with the hose due to its extreme cost, or perhaps was talking said cost up in order to start a negotiation (in the middle of a raging fire with a desperate neighbor).