Where is the tear? How big is it? I've ripped suit pants twice now (before I learned to stop carrying my wallet in my pants pocket). One was a suit separate, so I could get a replacement pair of pants. The other suit I had no such options and had to have a tailor stitch it up. The repair is noticeable only on a close inspection, which I do not have to worry about since it is in the seat of the pants and covered by my jacket or not visible because I am sitting down. If you cannot afford a new suit, I would talk with a tailor to see what they can do.
As for interviewing, bear in mind that, depending on the job, it could be worse to wear a sport coat and slacks than a suit with a small repair. As someone on the other side of the desk in an interview, I truly doubt I would ever notice a small repair in an applicant's suit pants (especially because they're sitting down on the other side of my desk within a second or two of meeting and shaking hands). I would absolutely notice, however, if the interviewee came in wearing a sport coat and non-matching slacks. In my field (law), that would be a big no-no for an interview. It may be different in your field.
