First of all, I wouldn't sweat this one much. Yeah, if the $50 numbers being tossed around are accurate, that's enough that it's worth thinking about before writing off, but put it in perspective. You were going to spend that $50 on beer---A beer---that's something of an extravagance. It's disappointing to lose it, but we're not talking about your last few dollars to feed your family. If it were me, the worst response I'd bother with would be not to engage this person in trades in the future. If attempts to agree on a remedy fail, I'd just leave it at that and move on.
Second, if you're looking for the best compromise here, here's my economist-style suggestion. Average the costs of the beers (NOT including shipping) on each side of the exchange, and call that the value that you each were trying to obtain. He got his value, but you did not. If he pays you half of that value---half the average cost---then you each come out of the deal an equal amount "screwed" by the unexpected event. It's not perfect, and there are other ideas about what is fair (believe me, I tried half a dozen of them before deciding this was my favorite), but it is simple and reasonable, especially since it's unlikely that one of you paid significantly more than the other.
Deciding on the value to be compensated is difficult, and where I'd expect people to disagree with me. I don't include shipping in this calculation. That is because you were each willing to eat the cost of shipping the product to the other guy in exchange for the beer. Even though your beer did not arrive, he still paid that cost already, so that part of the deal worked as planned. It's just a matter of sharing the cost of the lost product. Most likely the beer costs were similar on both sides of the trade---it's unlikely you were trying to send him either a $5 or a $500 bottle of beer in exchange for his $50 bottle---so the average is probably close. However, even if the costs were grossly different, simply averaging them is a decent compromise. At least, as good of one as I can come up with.
Finally, if you are still trying to reach a resolution in this case, unless he has admitted otherwise, I would graciously assume that he had done as good a job packaging as you did and ascribe the fault to FedEx. It may or may not be true, but you're not going to get anywhere by picking a fight by being accusative. In the future, in addition to agreeing in advance about how to deal with a lost or damaged package, I'd also agree on packing methods in some detail. Then, if you receive a package that doesn't meet those standards, whether intact or not, don't deal with that person again. It can't protect you from an outright scammer, but if you can't accept that risk, you shouldn't be swapping with strangers in the first place.