No reason at all to bamboozle the OP with suggestions of places miles away from St Pancras. It should not be a problem finding somewhere cheap within walking distance when booking in advance. One of the following mentioned above will do you nicely:
- Ibis
- Premier Inn
- Travelodge (several walkable from STP)
- One of the B&Bs on Cartwright Gardens, but do check to avoid a shared bathroom