I had someone tell me a while back $100 is an adult $20. As in, what you saw as a kid as $20 of fun but factoring in how long it took you to get the $20, it's about the same. I know that math will be massively variable, but I like the idea of equating kid brain money to adult money for "toys" like this. A $5 toy vs a $40 toy, as a kid, is a big deal. A $40 toy vs a $70 toy almost isn't, because they're both already "expensive".
I suspect for most on HN, their toy-money equivalence leans to the Radiacode or equivalent being the better buy.