Prices range from around $200 to $10,000.
Trombones are quite simple to produce compared to other instruments and are comparably cheaper. 
Just know that you get what you pay for. Don't buy a $200 trombone and expect to sound like a professional. 
Read all about the cost of a trombone hereWell, a case or a gig bag are nice to have if you plan on playing with others (and you most definitely should do that). 
A music stand is also nice.
You don't have to.
I have played trombone professionally for 20+ years but opinions expressed on this page are just that: my opinion.