There are several non-serviced apartments in town that rent by the month, but it still probably will be no cheaper for you than staying in a guesthouse for the nights you need, for such a short period, especially as you can check out whilst travelling.
Chansone apartments on Khouvieng Rd are $500 per month, but if you rent less than a month, even by one day, they charge $30 per day. They own two other apartment blocks just a bit further out of town, one at $400, and the other $250 but out at km7 (so you need to rent a bicycle/motorbike) .
The other option which may suit you is just off Dongpalane Rd at the Khouvieng Rd end. They have large bedsits (so one room, but with fridge and basic cooking facilities, wireless, ensuite bathroom) and charge $265 per month. Its new, and aimed at short term people that don't want a hotel. I have their leaflet at work - will forward email & phone details to you if you like.