to the right
users won't know what to expand until they read it, so the logical option is to add the button AFTER the text (hence, to the right). To avoid having rows with very short text and then rows with heaps of text, you can limit it in columns so you'll have all buttons aligned.
however...
the above answer really applies to general buttons. If you want to use a expand button, it's because you might have rows with lots of content and therefore need it. So, you should only use this when content is too long (and short text rows shouldn't have any button, of course). So, you might want to consider a more common approach, like a "expand" link right after the text that needs to be expanded. Like so:
this is a very long text. this is a very long text. this is a very long text.
this is a very long text. this is a very long text. this is a very long text.
this is a very long text. this is a very .... [expand]
In short: if possible, the best option would be no button and right after the text ends