Yes, a non-resident must fish with a guide every day that they are on Newfoundland waters. I paid about $130 per day back in the late 90's.
When you add up all of the costs of salmon fishing, it costs about the same to fish Newfoundland inexpensively as it does to fish any other salmon fishing location in eastern Canada.
In Quebec, a non-resident doesn't need to have a guide on most waters. However, you will probably be paying at least $70 (or considerably more) per day to access the fishing pools on the river of your choice.
In New Brunswick, a non-resident must have a guide (@ $100-300 per day). There is public water in NB, but it can be difficult for non-residents to get on good pools without help from a local. So, you probably will be paying a lodge or land owner for access to their pools on top of your guide's fees.
Nova Scotia is the economy anglers spot ... no guide requirements for non-residents and no water access fees on most rivers. However, their most popular river (Margaree) is quite busy (very busy in prime time). Most of the rest of the salmon rivers in NS are spate rivers and/or have limited fishing opportunities.
PEI ... I've not fished it ... Salmon Chaser can give all of the details. I believe that there is no guide requirement or water access fees in the province. However, your river options are quite limited on PEI.
Obviously, a guide will provide a major benefit to any angler fishing unfamiliar waters. I'm not advocating a 'go it alone' approach, just stating the legal requirements for each province.
The angler needs to travel to the location of their choice, and pay to stay, eat, etc. once they are there. For the economy angler, relatively cheap restaurants and motels can be found in every province. Travel might be a bit more expensive to the Rock than to NS or NB, but probably not too much more than getting to the Gaspe or North Shore of Quebec.
So, I figure that 'economy salmon fishing' ends up costing about $200 per day in any eastern Canadian province. One pays for different things in different places, but it all shakes out about the same at the end of the day.
I talked to several experienced salmon anglers that know their way around Newfoundland. They hired their guide, dragged him around to where they planned to fish and they did their own thing with fly and pool choices. Yes, they had to pay him. However, most guides will be flexible - if you want to use their expertise, they are ready. If you've got your own plan, they'll be quite content to accompany you and let you call the shots.
Unless the angler is already a resident in one of the Canadian provinces that is blessed with salmon water, the guide requirement won't end up making the cost of fishing in Newfoundland very different from any other eastern Canadian location. On the MAJOR upside, there is no private water in Newfoundland, they've got about 170 salmon rivers, the people are great and you will see some amazing pools. Oh ya, you will get fish if you go for a week in season.