Seems to me like the 5* hotels doen't really do the promos like the others which makes sense as if you're staying in the best hotel on the Strip, I guess you don't expect to pay $29/night...just a thought?!?
I think the most you'll get with Bellagio/Wynn/Encore is 25% off an advance booking of 60 days or more with the right promo which is still around $125/nt. Sometimes Mandalay Bay and The Hotel have a buy 2 get 1 free promo deal but I haven't seen one recently. Upper end hotels cater to a different kind of clientele.. Unless you're a high roller with deep deep pockets and get huge comps you have to put up some $$$ for a room there. Lucky for me that Vegas is in kind of a slump right now, I don't know if I'd pay the $300++ per night that these rooms usually go for. Hard to justify that when you can regularly get cheap rooms at many of the strip hotels and you don't go to Vegas to spend time in the room.
PS. If you're going for a few days consider renting a car - its usually cheaper than taxis. Get it directly with a rental agency (its too expensive through Expedia) and with coupons you can usually get a standard-midsize for around $15-$20/day.