Prices range from around USD 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 USD 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.