We have cruised on Mariner, and are again in September. From everything I have read, the prices are the same on all of the Voyageur class ships.
Johnny Rockets has an "upcharge" of $3.95 per person. That includes your tip. They have great onion rings and fries, hamburgers, sandwiches, milkshakes, sundaes, etc. We ate there twice, on port days when we returned to ship early. It wasn't busy at that time, but I have heard that it gets very busy on sea days. DH is a hamburger fanatic, and we thought it was worth the extra charge.
Chops is a steakhouse. The upcharge is $20 per person, including tip. They have VERY good steaks, different appetizers and desserts. It is completely the steakhouse experience. We thought it was worth the extra charge!
Portofinos is an Italian restaurant, and it is also $20 extra. We haven't eaten there, but may on the next cruise. They also have a Murder Mystery night in Portofinos, and I think it is about $45 extra per person, but that includes wine. I have read rave reviews about Portofinos.
If you want to go to Chops or Portofinos, I strongly suggest that you make a reservation your first afternoon or evening on board. Unlike the dining room, you can select a time that works for you. They do get booked up, and I have heard that you could wind up with a 9:30 or 10 pm dinner time if you book too late in the week.
Both Chops and Portofinos do not require a reservation on the first night. And their suggested dress is "smart casual", even on formal night.
On the Promenade deck there is a cafe with sweet rolls, small sandwiches, cookies, cakes, pizza, and coffee. There is no extra charge for this cafe. It's a great place to pick up a mid-morning, afternoon or evening snack. I think they serve pizza well into the night (3 am???). There is also a coffee bar (extra $$$) and Ben & Jerry's ice cream (also extra $$$) at the same location.
Hope this helps!