What is the Value of an 1885 20 dollar piece?

If there is no mint mark on the back you should have it authenticated by a dealer or appraiser because an $20 coin from Philadelphia is a key date. As of 02/2009 values start around $5000 for a moderately worn example and can go over $35,000 if uncirculated.

If there is a mint mark on the back its value will likely be much lower:

With an S mint mark, $1100 to $1300 in circulated condition.

With a CC mint mark, $1100 to $4000.