Value two and one half dollar gold piece dating 1850?

Its value depends on its condition and whether there is a small mint mark letter on the back. As of 09/2008, retail prices are roughly

No mint mark letter : $145 if very worn, $215 with moderate wear, $300 with little or no wear.

"C" : $250 / $1200 / $2100

"D" : $275 / $700 / $1900

"O" : $150 / $300 / $575