I know of a few places that you could try:

There is one store that has a few second-hand bikes (plus some new bikes and some mopeds) halfway between Place de la Rippone and Place du Tunnel. Those bikes tend to be pretty good, in the range of 300 CHF and much more.

The salvation army (Armee du Salut) normally has a couple of VERY basic bikes hanging up in the back, normally for about 50 - 100 CHF.

I recently saw a flyer about a place up in Cheseaux called Cycles Ricca. Here is a map, and here is a link to their website, which says that they have used bikes from 150 CHF and up.

You should also try looking at the noticeboards at the universities (Uni' Lausanne and EPFL).

Let us know if you find any other places.