Let's assume that you have 40 years left to live. As a starting point, let's also assume that average annual inflation rate will be 2% over that period.
You can figure out how much $1 in 40 years is worth today by doing simple present value calculation:
PV = FV / (1+average annual inflation rate)^# of years
Now plug in the assumptions,
PV = $1 / 1.02^40 = $1 / 2.2080 = $0.45
This means that in 40 years $1 will buy you the same amount of goods and services as $0.45 buys today, i.e. you'll be able to mail a letter in the US.
remember these are a guess using these specific numbers hoped this helped