A sovereign is another word for a gold piece, essentially worth 100 silver pieces, which in turn are worth 100 copper pieces each.
If you want to get this legit, it is obtainable by completing every quest possible in Act 1, so long as you don't buy anything from merchants (buy what you need after you have this achievement).
When I did this, I mostly went for the 'good' choice at the end of each quest meaning less or no reward, and I was still able to scrape through by doing everything apart from the "Friends in low places" quest. You can make a save and sell all of your equipped gear if you're not quite there, just remember to reload after the achievement pops. If you still haven't got quite enough, you can also roam the streets at night to grind random bandit encounters, although you will only earn a little money from each one.