I get that this is a joke. However, technically speaking, even if, say, that the $8 here is now worth $11, the quote would still apply. Why? Because if we make this into a mathematical notation: (fun things) ≥ $8. This means, when (fun things) ≥ $11, the original assertion still applies - that is to say, when something costs at least 11 dollars, it definitely costs more than 8 bucks.
Now, yes, I agree that, to a certain extent, the specificity and certainty of the original saying become decreased. That is, under the unmodified version, things that cost $9 could be fun, whereas using the modified one, we know it cannot be. However, this is not much big of a deal. First, there is not that much harm that comes when we think something can be true but ends up being false. Second, this is not an exclusive problem. (Fun → costs at least $11) is not the equivalent to (costs at least 11$ → fun). In other words, there would still things that can be considered potentially fun under the modified assertion, but actually isn't. For example, a paperback version of Moby-dick costs $13.45 on Amazon. However, we all know that it would be a boring read.
There's nothing you could find, but I'll give you a hint: many people have read his biography.