My husband and I just went on a cruise and we too were on a tight budget. I was worrying about having enough money for tipping. It was starting to stress me out, but in the end it turned out fine. We chose the automatic tipping option where they just added it to our sea pass. It ended up being 39.00 per person for the length of our cruise which was about $10.00 a day. On the second day of our cruise they passed out a flyer in our stateroom that gave us the option if we registered with our credit card to have the tips automatically taken out....if you don't choose this option they still leave in your room envelopes and on the last dinner you have you can hand your waiter and assistant waiter the envelope with your tip in it.
Your sea pass will add up quickly if you order soda's, or drinks from the bar, but if you can handle drinking water, lemonade, or tea which are free, or when you get off the boat you can shop and pick up soda's or bottled water at a much cheaper price and bring them back to your room to enjoy.....if you can keep your sea pass total down then at the end of your cruise your bill will not be high at all.
I hope this helps, enjoy your trip