For my month-long Bangkok respite in May and August this year, I opted for an apartment in Ideo Mobi condo, within walking distance from Phra Ram 9 (Rama 9) MRT station. It is definitely much cheaper than if I were to stay in a hotel for 30 days and I had the entire apartment all to myself. You can search for this condo on airbnb.
I would recommend this area to stay because it is very convenient. Just below the condo is a 24-hour supermarket. Across the street is a super huge 6-storey Central Mall and diagonally opposite is the even bigger Fortune Town, replete with a row of good massage joints and a huge hypermarket at its basement. There are all kinds of food available, from foodcourts to restaurants offering every cuisine you desire.
Within the condo is a library with free wifi (above), a coin-operated laundry (30 baht per wash), a fully-equipped gym which I never stepped into, and meeting and game rooms you can book to hang out with friends.
Much to my delight, there's also an under-utilised pool that I do my laps daily.
Going to and fro the airport is also very easy. Just take the train from the airport to Makasan Station and transfer to the MRT station Petchaburi. Alight one stop later at Phra Ram 9.