Edit: July 6 -- Fixed In Update

There's a thread in the problem reports forum about this, but I figure it's better if the workaround is clearly labeled as such.

The issue is that when you buy a favor item in single-player games, the offline favor points are spent but the item is not saved.  Thus, players who regularly play offline or test builds with certain items will quickly run dry on points as the same items must be re-bought and most cost more than what you'll earn in a match.

The workaround for this bug is--

Go to your Documents directory (on Vista; My Documents in XP) and open the directory My Games\Gas Powered Games\Demigod.

There's a file called Game.prefs.  Simply open this in a text editor (e.g. Notepad, Wordpad, or the best, Notepad++).  There will be a line that says Points = [some number], with the number being your favor points for single-player.  Use Ctrl+F to search for "Points =" to find it immediately.  Just change the number to add back the extra points you're spending on the same items (or just cheat and give yourself 9999 if you really want to) and save it.  Remember to keep the comma at the end of the line.