Audiobooks are not a conversion. T are separately produced using a chosen voice artist, who is recorded reading it. It costs serious money to make an audiobook unless you have studio equipment, time and skill to make it yourself. Not to mention a voice people will want to listen to. The best way is to have the book sell so well one of the production companies offers you an advance to make it and distribute it for you. Alternately there are companies which will make them, but you can pay anything from $100 to $500 a finished hour depending on who the voice artist is. As a rough guide, every 10k words will require 1 finished hour. Even an average length novel will cost somewhere around $2000 to have made.
Of course, you can get audiobooks made for much, much less but the cost of recording can add another 1500 to 3000. Of course, one can spend tens of thousands of dollars on voice actors and even hire a professional to perform the audiobook. But the audiobook will have to be a licensed and approved product and all royalties go back to that company which created it. The cost increases dramatically if there is a licensed character or world, but you never know how much one will cost. You can find out by talking to your producer! (It's a lot like a lawyer!) A voice artist will spend around 2 years working on an audiobook after it is done, usually 3-5. This process usually includes re-recording the audiobook once, which is then converted into a digital file and sent to the voice actors. Once they have spoken, it is converted.