Cheap alternative to glass?
Soft pastel is literally the most difficult "painterly" media to protect. Generally speaking, the appropriate way to protect pastels is with a so-called shadow box, where the paper or board is floated above the backing plate of the frame and still has air-space to the glass. (See my drawing below.)



This is a pretty material-intensive method, but it serves to isolate the pastel from touching anything, including the glass. You have probably noticed that applying fixative to soft-pastels will change them, dulling the whites etc.

There is, in my opinion, no cheap way to protect pastel drawings, but I do have two suggestions.


Find cardboard boxes with clear plastic inserts, like gift boxes where you can see inside them. 
Take a high resolution photo and print them at the local drugstore or at an online service. Frame them as you would any photo.


The second option is probably the only way to cheaply conserve your pastel.