Neat question and, for the life of me, I couldn't remember what an expensive sweater cost in the early '80s when I first started shopping at BB. To be fair, at that time, I was all about the least-expensive, on-sale items, so $150 sweaters were way off my radar anyway.
So, I tried to find the cost via an old on-line catalog, but the prices weren't in them, at least on the page with each item (which is not how I remember it, so it shows you how your memory plays tricks on you); of course, there might have been a "price index" page, but I don't know as each page of the catalog was loading slowly and it was 300+ pages long, so I didn't go through the entire thing.
Then, I went to an inflation calculator. Inflation calculators work off of a generic "basket of goods," thus, they are a guideline not a precise measure. Here, I took a BB cashmere cable knit price from today (as I am guessing that the sweater in '75 was a cashmere one) - a full price of $498 (on-line) - and that comes out to $102 in 1975.
Hence, recognizing that, as noted, the inflation calculator isn't precise - and I'd say clothes were more expensive relative to other things back then (i.e., they experienced less inflation in the last 40+ years than a generic basket) - $150 doesn't sound crazy. But, to emphasize, that's not based on anything more than an inflation calculator, the real price could have been higher or lower. I'm sure, someone at AAAC will have evidence of the real price.
And all that said, I love the story (and want to know what happened to the girl and her generous father?) and, if by chance, gr8w8er memory is off a bit on the price, my guess is that $100, $150 or whatever was such a crazy big number to him then, that he, over 40 years, might have gotten the exact number confused in his memory.