I'm with Ira (comme d'habitude). The XIVe around Gare Montparnasse has a lot to recommend itself:

Montparnasse has a "village" feel rather than being just another city neighborhood. It's almost like being in a provincial town.

Better lodging values than in the single-digit <i>arrondissements</i>.

Well-supplied with transit options - M&eacute;tro Lignes 4, 6, 12, and 13 converge at Gare Montparnasse (and Ligne 10 serves nearby Duroc), so you can be virtually anywhere in the city in ten minutes.

Wow! Thank you all for your suggestions! We're trying to keep our budget to no more than $150 euro, our pockets aren't very deep. :)