I've read older posts here and in some other forums that people have had issues using Bricksync's loadprices command with their bsx file and just wanted to post my solution in case anyone else might need to fix theirs.
First, edit your bsx file in Notepad++ or similar text editor and make the following changes:
Remove the currency attribute in the <inventory> tag. (This has been the usual advice, but it didn't work for me, so I went on to try the additional edit below.)
Now, go to the bottom and after the closing </inventory> tag, you'll see some additional fields that BrickStore adds. Delete all of those after the </inventory> tag and you should be good to go.
Just remember to put your full pathname for your file in the command if it's not in Bricksync's current working directory.