what was a coat of arms valued in the middle ages?

Answer 1

A coat of arms validated one's claim to the nobility. This would entitle one to own land, collect rents, act with considerable freedom, participate in battle (a primary way to earn respect, and money), and generally to avoid the unpleasant aspects of life as a free-man, or serf.
Based on that, it was extremely valuable, though since the economy of the Middle Ages was primarily a barter economy-any comparison with today's value is not possible.

Answer 2

A coat of arms did not realy have a monetary value - esp. a consistent one. They were often granted and inherited. The right to bare arms (not like the modern idea! - Heraldry - not weapons - though it is linked!) often (but not always) came with owning enough land to earn rent to a certain amout (that varied over time). So you were expected to be at least reasonable wealthy to be an "armiger".