What is the value of a rare 1945 gold walking liberty half dollar with diamond?

It's not gold, just gold plated, so it's not all that rare. The US never made any gold half dollars because at the time a gold coin that size would have been worth about $35, not 50 cents.

Based on the diamond, my guess is that it was once part of a pendant or locket, which is why the coin was plated. The underlying silver is probably worth $5-7 depending on the coin's condition before plating. The diamond would require in-person evaluation by a jeweller to determine its value.