You could always try the Holiday Inn in Longueuil (1-800 Holiday), although the location is not as good and I can't comment on the hotel, having never stayed there. Downtown Montreal hotels are jacking up their prices too for whatever reasons. If you're really want to get quick on the Rive Sud, you could stay at the Gouverneur Hôtel Place Dupuis? You won't have to go out for the subway and the yellow "Métro" line to Longueuil starts at BERRI-UQAM where the Gouverneur is located. They tend to have good rates too, especially on weekends (with full breakfast buffet for something like 139$-149$CAN
for two) and they will always try to sell you their renovated guestrooms on floors 22-27 for an extra 15$ but in my opinion, they're not worth the money as standard rooms have been remodeled in 1998 and are very clean. Be warned though that it's the entrance to the Gay Village (so not for homophobes for sure), but it's a large hotel, they have all types of guests (lots of families, Gouverneur is very well known here in Quebec, corporate travelers, etc.). You can also check their website http://www.gouverneur.com
That's about the only one in downtown Montreal with quick access to Longueuil.
Hope this helps.