You'll wear it for interviews, when you want to look your very best. That alone should warrant a nice suit . I'm not talking a $1000 suit or anything; regardless of how much it costs, it should be made of a quality fabric, and fit you perfectly.
But after interviews, you may end up wearing one every day at work, depending on what kind of job you get. Firms and D.A.'s offices are all different; some allow business casual, others want business formal.
If you live near a Filene's basement, I've heard they have a decent suit section. K&G Mens usuallly have a huge selection, and either free or reasonably priced tailoring.