Heaven@4 Hotel on Sukhumvit soi 4 has a stay 4 nights and pay for 3 discount again. The hotel is in a great location and I've written a post here with a map showing where all the hot spots are nearby as well as the guest friendly confirmation.
So the deal is book 4 nights, pay for 3 and each night cost around 1125THB, just a little over $32 bucks. As you can imagine it's a really popular hotel and even though it's only 3 years old it's seeing some signs of wear and tear already. But because of the value and location I think it's still worth staying there. It's a limited deal so if you're coming to Bangkok soon you should book ASAP.