We are on a budget too. I am a stay at home mom, so we are a one income family.
One of the things I LOVE about the cruise is that when I have it paid for, there is not much else to worry about.
We didn't do any of the 'extras'. We took some pop with us for in the room. So we didn't order drinks (well, OK, I splurged one time and let the girls have smoothies once

).
Tips were my biggest expense, but I knew they were coming.
We had a blast! We didn't feel we missed out at all. There is plenty to enjoy without doing the extras.
Now, in September, we are going on the 7 day cruise. This is a BIG splurge for us. We want to do a couple of excursions since we know we will never do this again (just can't afford it....we can do the '3 Day Cruise', but the 7 day isn't as easy for us to pull off).
You will have a great time (and will be so glad not to see a big bill at the end---those drinks can add up, and I talking about smoothies and pop, not just alcohol). The beverage cart by the pool and the pop I brought for in the room was plenty for us!
Debra