If you have flown with any airlines that fly this route you can check their promotions that are sent to phone no/ e mail address (if you have flown them before) or do so on web. Probably these sell out quickly.
I don't think that the price you quote is expensive - several of my staff travelled to and from Mumbai, Bangalore and other locations in india to Udaipur and Jaipur many times last year and I don't think any fare was less than 4k 1 way including add ons, some mandatory; ocasionally 15 k from or to Kolkata. Also you have to consider whether it is worth saving a few INR and having less time in Mumbai.