once it's been handled, a proof doesn't keep much value. Proof sets are so plentiful that collectors don't typically want "circulated" proof coins.

I suggest you take it to a coin dealer in case it is something rare (such as an error, since you don't see an copper strip), but even if it was an uncirculated proof, a single coin be worth $1.50 - $3 or so. Sometimes silver coins are made at the Philadelphia mint (I believe the national parks series has done that), so maybe someone with expertise can determine whether the Denver mint could have made silver quarters when yours was made.