You should ask what is the average upkeep of the MH and a toad vs a FW and the tow vehicle.
It also depends on how much you are going to use either. Are you going to be stopped a lot? Will you be travelling a lot? Once parked do you mind paying for a lot of gas for the tow vehicle? Do you mind driving a one ton truck as daily transport?
The MH, as a road vehicle, requires more insurance. There are some unique ways around this one, but, only in certain situations.
Then there is always the gas vs diesel issue on long term cost.
We have owned both and, in my openion, the 5th wheel setup is less expensive. The 5th wheel itself was very low maintenance and the 1 ton duely deseal, was more frequent but standard vehicle maintenance. The motorhome is by far, for us, a better traveling vehicle but motor, transmission, etc. on a big rig is, less frequent than a truck, but a lot more expensive. The toad, just general maintenance with added tire and barrings maintenance because it's towed.
A few years ago I had someone ask me that same question. I thought about it for a bit and then told him, Pleanty and that's only an estimate and will probably grow from there. Come to think of it now. I didn't know how I could have been more on the truth. It's just like a house, do nothing and it will run down, costing a fortune to repair. TLC and it looks and function just as it should. It's just a constant maintenence like anything else with a little more to boot. Remember you're putting a house on wheels so the problems can easily multiply. Some people may not do the constant maintenence and trade their unit often. Then others stay on top of things. Just MHO Seen it done both ways.
My wife and I are researching this very subject as we near retirement and want to FT. Both have advantages although the MH weighs in more expensive from what we can see. We were set on a 5r with a big truck to pull and then we went into a MH to look at floor plans. It was so nice inside and set us to thinking. We will be looking at each other every day (not a bad thing) and would the expense of a MH be worth it. We came to a very tentative "Yes". That said, we are still in the research stage and have made no solid decisions. There are some nice 5r's out there and a good one can be obtained significantly cheaper but the end all, in our estimation, will be the quality of life when we f/t. We have set our 5r up in the rain and snow as our neighbors came in and auto levelled in their MH, right in front of us ???. We have also been playing with the up front costs, insurance, fuel mileage, toad expenses and PM which are all higher for the MH but not so much that it cannot be done within our limited budget, esp if you buy used. Both seem to be a good way to go and the bottom line for us will be budget followed by quality of life. Good Luck! Phil