PF-61 or PF-62 would be generous, I don't see enough rubbing really to justify bumping it down to AU, but its certainly in the lower grades.
Its not bad for being circulated, but proof coins pick up scratches and scuffs a whole lot easier than circulation grades. With the same handling methods, a business strike coin might grade within the mid MS-grades, but proof coins are very easy to damage.
When you can pick up a PF-68/69 example for $10 graded, its not really worth sending in. Of course it only costs 5 cents to keep so by all means keep it for your collection, but its a fairly poor proof example.
To be honest it isn't that great, flawless proof nickels for that year can be had for about 3 dollars and that is for already graded PF69DCAM, ungraded flawless ones can be had for a dollar. Not sure of the grade but from the pictures there is some wear but those scratches really hurts it.
The first thing to remember is that proof is not a grade but a method of manufacture. Since it was found in Circulation, the best I would grade it would be au58. There is enough rub on the jaw, cheek and on the dome to make it down to that grade if not more. Given the rarity (I.E. no rarity). I would put this at $.25 to $.50. Still, for $.05, it is pretty cool and a could make a nice gift for a kid who was just starting out.