I have -8.5 eyesight. Is it curable with eye exercises and good diet? Please tell me the cost for laser

You have a very severe case of myopia that most definitely cannot be cured with any alternative or low cost methods. The cost of Lasik depends on your local provider and you must go in for a consultation in order to get a Lasik price quote. I also recommend considering a contact lense implant such as a ClearLens Exchange in your case. The cost of a ClearLens is more expensive but will surely correct your vision. Lasik can run anywhere from $2,500-$4,000 and the ICL procedure can cost $3,500 to $7,000 depending on the laser used in Lasik and the lens in the ICL.

Unfortunately, eye exercises and good diet will not make your vision clear. Luckily, glasses, contacts, and laser correction can improve vision. The cost of laser treatment may vary based on the type of correction (PRK, LASIK, and SMILE) and technology. If you are interested in pursuing laser correction, please schedule a consultation with a doctor to ensure that it is safe to do so. 

Unfortunately there are no eye exercises or diets that will improve your myopia/near-sightedness.  Glasses, contact lenses or laser vision correction (LASIK/PRK) are your primary options for clear vision.  You would have to be evaluated by a LASIK surgeon to ensure that the shape and thickness of your corneas are appropriate for the procedure and that your prescription is stable.  Everyone's pricing is a bit different depending on the technology that they use but in my area it is typically somewhere between $4000 and $6000.