Jetstar upgrade - seriously?

We fly to Bali in 3 weeks. On the website the price of a business class airfare is $722 per person vs economy fare price of $502. A difference of $220 or $1100 for the five of us. When I asked jetstar if we could upgrade, I was quoted $4980!! What the actual? I asked why the difference is so high and was told it was due to change fees??

Indonesia-Denpasar (Bali)