Don't do it for the money!
I made enough to never want for anything while on board (tons of shopping while in port!) and very comfortably get through 3 months vacation with a fair amount of travel a year, but you're not going to come out way ahead or anything. I did pay off quite a bit of credit card dept my first contract, but after that I spent a lot on stuff on shore. My first world cruise I assumed I would never see those places again, so did everything I could. Then did another where we spent a 4 days to a week in all the major cities we hit, so spent lots on hotels and tours.
Brill way to meet people and see the world though. You have no bills, you pay $1/day to the person that cleans your room, other then that you could save everything if you liked.
It's like a non stop party for the crew (think the staff area in Dirty Dancing. We called it Dirty Dancing syndrome when passengers would try ad sneak down to crew area) and you make some of the most unbelievable friendships. I went to a friends wedding in Italy a few years ago, and this may there are about 10 of us (from Australia, England, South Africa & the States) spending a week in Vancouver. I actually just spent the last hour texting with a guy I worked with 7 years or so ago in Alaska.
I've had a wee bit of wine and will keep babbling forever, so will stop now. But if you have any other questions, feel free! It was a fantastic experience and I love sharing it