You were right to say there are many different speding styles!
If you want to keep the costs down, then the best thing to do is know what is out there to buy and prepare the kids (and yourself) before you go. You are lucky that all the basics are taken care of, so it's *your* decision how much more the trip will cost.
For us we keep it pretty simple - this is what we did when our kids were 5 and 3 (close to your two) They were each given $20 to spend on souveniers for the week, we explained it before and they understood they could always come back later and buy something (so they didn't spend it all the first day). We then bought each girl a tee-shirt ($15) and gave them a smooshed penny book ($6) (you fill them with flattened pennies - 50cents a smoosh - we probably did about 5 a day) They were told they could choose one penny design at each machine (they are in all the shops and major rides) So total kid budget for week - $87
Beware of the laynards! They are very cool, but can bust your budget! The laynards are worn around the neck and then you buy or trade pins to place on it. A starter set, laynard and 4 pins is $25. We brought laynards from home and then the girls used some of their $20 to get it started. (I think pins range from $4 - $10) When our kids were younger - we just said no and they stopped pestering us. But they are very addicting
I have one and add to it each year.
We usually get my husband and myself a shirt ($40).
Costs that could add up would be alcohol and breakfast. A muffin and drink will cost you at least $5 a person. Having cereal in your room - a lot less. Will you need gas money or are you doign Magical Express? It's all about choices!!
So for us, we would need about $250 - $300 a week if we were on the eating plan. We go about once a year, so the merchandise doesn't call to us as much as it used to. But if you want to splurge - go and have fun! We have done it both ways (watching costs and not) and it is a great trip both ways as long as you have your expectations in line before you go.