The Senior National Access pass that most of us cherish is available when you turn 65.
As far as comparing cost....I know that others will throw in their two cents worth, too. I am not sure there are ways or sites to compare? So much depends upon so many different personal choices and variables, many of which you may or may not even know how to identify yet, without more experience on the road. Just a few variables to consider:
How much of the time you will be moving down the road and consuming gas/mile vs how many days you will be sitting still.
What sort of camping experience do you need? Full hook ups with a pool and hot tub available to boondocking out west on BLM lands.
Rigs come in all sorts of sizes and ages and conditions.
How much maintenance and repairs can you do yourself? Some of these women are very talented at completing their own refurbishing and repairs. Many of us pay for the repair talents of others.
Food expense is food expense on the road or off unless it involves eating out more often....
Visiting historical sites, museums and other activities can add expenses quickly.
Do you plan to work to earn money or RV sites? (Work Camping)
Be sure to think of the expenses that remain similar whether or not you live in a S&B house or one on wheels....like medical and insurance, vet bills, etc.
Think of what you want or need in the way of an "emergency" fund for repairs and unforeseen happenings. Again that safety net is no different no matter where we live and depends upon an individual's tolerance for "risk" taking.
And then there is that great unknown of how you might adapt to the life style. Often many of us have found our needs and wants change with our experience and changing priorities that only happen after we have been out traveling awhile....