
It depends on the bike. Once you've found a bike you're interested in, call
some insurance companies for rates. Some bikes are much cheaper to insure than
you might think (my EX-500 only costs me $127/year in St. Louis; I'm 27)

For a first bike, I recommend 500cc's or less. And a bike short enough to get
both feet on the ground when you stop.
 
The one piece of advice EVERYONE will give you is to take a Motorcycle Safety
Foundation Rider's Course. See a dealer for details. In some states, completion
of such a course can give you a break on insurance. It will also teach you
to ride properly from the beginning, so you won't learn any bad habits.

Good luck. Riding a motorcycle is the most fun you can have, naked or otherwise.