Now I could be a wiseass and say .....
H-O-T-E-L-S.C-O-M ... but I won't.
Of course it all depends on what you mean by "cost effective".
Usually I go for HDLM (Hotel de laMontagne) just because it's very convenient to everything on Crescent St. & St Catherine; it's got a rooftop club & pool where it's great to hang in the summertime; is attached to the bar/club Thursdays via tunnel; has parking underground on premises; spacious guest rooms; large bathrooms w/ tubs; and many rooms have balconies.
And also, as many will attest to, it's very SP-friendly.
But it's rated somewhere between 3 to 4 stars depending on which website you're talking to. The cost will range from $100 to $200 per night depending on the time of year & what's scheduled that weekend as far as festivals or special events.
For the $90 to 130 range you can get a "suite" (and I use that term loosely) at Le Cantlie Suites Hotel just a little further up on Sherbrooke, near the Omni & Ritz-Carlton. Looks like it was built in the same "cast-in-place" concrete era as the HDLM from the outside. Inside not as fancy as HDLM so it's OK if you're not looking to impress anyone. Seems more 'family-oriented' so SP activity should be very low-key. There's parking down below but not as convenient (access through back alley) and they've got a rooftop pool, too but with more kids & alot less eye-candy than HDLM.
But for "cost effective" the plus is the suites have a studio-apartment style galley kitchen. Not just a kitchenette unit but full size appliances including oven & stove. So if you want to cook for yourself (and guest) or just be able to keep leftovers for another meal that's where it can pay off. Buy some bread & cold cuts for sandwiches and make your own coffee & breakfast in the morning instead of heading out to Second Cup or Eggspectations.
But the one thing I've always really liked is the fact that the "bedroom" area is separate. It's not really a room - they just sub-divided a large room by adding wood & glass partition with French doors. But it's a nice touch to be able to go from sitting on the sofa to taking a woman by the hand and leading her away to the bed. Other than that the place is pretty generic as far as hotels go. Better than HoJo's, not quite a Hilton.
P.S. - The thing to remember really is the more you save by staying further from downtown you may end up paying in cab fare if you plan to go out at night. That's the economical part about staying in the middle of everything. Even with a car it's cheaper & easier to take a taxi than to deal with paying for parking, tipping, possible parking tickets if you're on the street, not to mention DWI and trying to navigate through an unfamiliar city with a buzz on!
Sometimes the extra cost for a convenient location is worth it.