I'm with a middle school team thats gone 3 of the past 4 years. Its a fantastic experience, as no other competition comes close to the level of intensity and the feeling of something that large. We have gone to the US open 4 of the past 4 years. Thats a big event, but still has the feeling of a local/state level competition.
Last year for US Open, the team had funds to cover the registration, but travel and personal expenses were covered on our own. We are in Michigan and drove, so for me it was 4 tanks of gas ($175-200), a hotel for 4 nights (varies, but should plan on at least $100 per night depending on your tastes of hotel), plus food. For food you can find a hotel that has breakfasts, and if you get into the Country Inn (part of the convention center complex), it easy to provide outside lunches and dinners at a much cheaper rate. We've brought crock pots and had some parents leave the venue an hour early, commondere the lunch room in the hotel and make a healthier option of sandwiches, salads, sloppy joes, taco bar etc. This at least keeps the food cost minimized. We all pitched in for food costs and its much lower than either going out of the venue or paying for food at the venue. This year we did not do this and we opted to go out of the venue for meals. Still pricey but better than pizza at the center. If you make the meals yourself, i would think you could do it for $50-75 in meals per person. If you go out, I would say plan on 20-25 per person per day, although pizza gets boring day after day.
Worlds was about the same. We drove (much shorter distance, so gas expense was lower), and hotels seemed a bit higher. I paid $150 a night for 4 nights. This year we ate at the venue, as its harder to go out and get back in time for lunch. I opted to go light and instead eat a larger breakfast (free) at the hotel, and then dinner to control costs. But, its easy to spend $10 at worlds for lunch at the venue. In the past, we did the same thing and some parents made lunches and brought them and we just ate outside.
Overall, gas, hotel, food... I would plan $150 per day minimum.
Souveniers... our girls liked the shirts at worlds better. shirts are $50-100 each, depending on what you want put on them. There are water bottles and other items for cheaper, but everyone went with shirts.
If you opt for other extracirricular activities, that makes the price go up. We have been too tired to go places after the event, so usually the girls just went to the hotel pool after dinner for an hour then to bed. The days are long and high intensity.
One option that saved money for a couple of us this year was getting a bigger house on AirBNB, and have multiple people stay there. They had a whole house and kids got a chance to wind down somewhere and parents sat on the patio with adult beverages...