While it cost only $3.50 to get in, you had to pay for those A-E tickets separately. So a true cost comparison would have to take into account the amount of money you'd have to spend in 1971 to ride everything you wanted to vs what it cost for one day (unlimited rides) access to MK today. To ride everything, once, add around $15.75 (source: http://allears.net/tix/tixpix70.htm ) or $94.55 when adjusted for inflation. Add that to the cost of general admission you're at a total of $19.25 or $115 when adjusted for inflation.
So today's one day pass ranges from $99-$119 depending on the day you go. But there is more to do, and you can do things as many times as you want. So really, the cost, as far as I can tell, is the same. It just sounds better back then.