Hi: First off, we'd have to know what model you have. It's printed on the ID plate, before the serial number.
As for value, that's a very tricky question. The three main variables for estimating a machine's value are Rarity, Desirability, and Condition: for example, a rarer machine that is less desirable, and in poor condition, will sell for less than a more common and desirable model in mint condition. A good rough guide is to look at sites like Ebay, and search for machines like yours which have already sold. The final selling price, all things being equal, will give you a rough estimate of what yours might bring.
The market for phonographs is fickle, though: the only reliable indicator of what price a machine might bring is, "whatever a buyer is willing to pay for it". I hope this advice is of some help to you.