I have no idea exactly what you are looking for or what your price is, but this page on the agoda booking site lists quite a number of very cheap hostels. I have to add i know nothing about any of them. I believe there is one at the end of Sathorn Soi 1 at the junction with Soi Ngamduplee which is pretty central. It used to be called S1 but then closed. I think it may now be called SoHo.

https://www.tripadvisor.com/SmartDea...tel-Deals.html