-what is/are the advantage(s) of cotterless over cottered?

Lighter weight and ease of maintenance. R&R of cotter pins to service the bearings is a PITA. And while not technically a benefit of going cotterless, you'll probably also gain the ability to swap chain rings a lot easier as it can be hard to get chainrings for those old cottered cranks.

-how easy is it to switch over, if you have cottered cranks?

It depends. If it's a British or Italian thread bottom bracket it's simple - just replace the bottom bracket and crankset. If it's one of those UO8s in your sig it'll be a little harder as the BB cups are French thread. It can still be done. I did it and it worked great on my UO8.

-how much would it cost, and is it worth it?

Again it depends. I'd need more information on what kind of bike and what you want to achieve by doing it. For me it cost about $20 for a new BB spindle and a 2mm shim plus a couple of hours of my time. I already had a crankset in the spare parts box.