Interpolating Krause Values

If the only value listed in Krause is for a proof coin, then it's most likely that the coin was only struck in proof, so whatever the coin looks like, it actually is a proof. "Proof" is a method of manufacture, not a grade; if it's been damaged or circulated then the technical description is an "impaired proof", but it's still a proof.

Valuing impaired proofs is tricky, since they don't tend to happen very often. I found an impaired proof Australian dollar coin in change a year ago (see this thread). I believe a good ballpark figure for an impaired proof is half to a third of catalogue value, though it does depend on what proportion of the catalogue value is it's bullion price. If the CV is close to the bullion value anyway, then BV is as low as it will go.