Hey there Cfmct, well since GKL got me thinking, and since I'm the forum "devil's advocate" kill-joy, I can't resist :
a) The "$50" rule that you speak of, is not unique to just that park.
Every single one of the 50 states have Lost & Found laws, as you know. Which set a value of somewhere between $50 to $250 value (some arbitrary #, that is long-since out-dated). To which if you find something,
you must turn it in to the police. These are born out of wandering cattle laws of the 1800s. And serve a good purpose so that no one can "find" the Mountain bike by the park bench. Or "find" the $1k Apple phone on the bus bench. Or "find" the wads of cash that fell out of the back of the Brinks Armored car wreck, etc....
b) And md'rs have frequently felt that coins are NOT included in those laws. Since, of course, for example, a $5 gold piece is (drumroll) only worth $5. Right ? After all, it's the stated value right on the front of the coin, right ? Or the 1916 D dime, in mint condition, is only worth .10c. Right ?
But GKL brings up an interesting point. That, while it seems like "grasping at straws", has actually come up before ! Ie.: Where/what is the criteria ? Instrinsic value ? Face Value ? Collector value ? etc....
There was an interesting case in a town near me, where an nerdy 3rd grader took his dad's coin collection to school for show & tell (without his dad's knowledge or say-so). Then at recess, began to pass out the coins to fellow class-mates to "make friends". These kids, in turn, (who think they're like play money or something) promptly go out and loose them in the sand-box playground area.
Ok, if you find these old coins, do they belong to the Dad ? Or to you ? Are you under any obligation to seek-out-the-owner, since, of course, they are worth well over the value of seated halves, trade dollars, etc... ? If you are the coin-collector dad, do you feel that the coins belong to the lucky md'r ? Or to you ?
Yes, I realize that this doesn't address the 99.9999% of gold coins and seated halves and trade dollars that we md'rs will ever find. But ..... if we're going to "knock ourselves silly following every law", then are we obligated to cross-this-bridge ? I mean, heck, what's the worst that can happen ? You turn the $5 gold into the police. And after 30 days, if no one claims it, you get it back, right ?