If the quarter was dated 1958, it was 90% silver, containing roughly .18 ounce of that metal.
If it had been "traveling" 22 years, that would make the time in the clip 1980, in which year the film is set.
In 1980, the average price of silver was $20.63 (from an intraday high of $50.35 on Jan 18, 1980), which means that quarter was worth about $3.71, on average, during that year.
The US Gov't stopped producing silver quarters for circulation in 1965 (or 1966; the records aren't entirely clear), but here's the trick: they had been continuing, on an emergency basis, to date these quarters (and dimes and halves) 1964. This was in "violation" of Federal law, which said coins had to be dated the year they were struck, but the mint was under orders from Treasury, and they set that aside for those years. Not relevant, but interesting. The takeaway is that all US quarters, halves, and dimes dated 1964 or earlier were silver. The Mint also introduced the cupro-nickel clad coins, which contain no silver (dime and quarter) and halves (40% silver until 1970.)
By the mid-70's, it was difficult to find silver coins in circulation. By 1979, due to the spike in silver prices, it was essentially impossible.
So, I doubt Anton would have given that convenience store owner a 1958 quarter as a quarter in 1980, and I don't think he would have been unaware of its value.
Or....maybe he did, and that was what the shopkeep won.