Yes! You can switch tunes at the switch of a button! In my Integra, I have a 91 octane tune and a E85 tune. My friend has a 'low boost tune' and high boost, though he has to switch that in his laptop.
Keep in mind my answers are purely with my experiences, it may be different for another brand.
Cost of the Engine management system installed and ecu: $600. hondata.com
Cost of the tune: Average dyno rate is $10o/hr, tuners charge anywhere from $100/hr to $500 per tune.
Servion Tuning
There are many benefits to having a custom tune, OEM tunes are very conservative and 'safe' so you can get some power or efficiency I am sure. Is it worth it? For mpg's, no way.
You can 'share' tunes, though someone elses tune that works is really just a fancy basemap and I would still recommend having a one off done for your car.
If your vehicle is modified you will probably get more out of your tune.