There are a lot of ways one could attack this problem.

One could simply account for the average rate of inflation over that time. Or you could use changes in CPI (I don't know how long this figure has been in use).

Or you could compare the average salary of a person in 1935 to the average salary today.

If for example the average worker made $10 per week in 1935 and today the average worker earns $400 (I am just making up these figures) one would calculate that a $2.50 bet in 1935 was the equivalent to a $100 bet today as percentage of the average salary.

Or you could use another index. The cost of a loaf of bread. If a loaf of bread cost $0.05 in 1935 (And when I worked in a supermarket I had many customers who repeatedly told about how cheap everything used to be.) and now costs $1.50 today then a bet of $3.33 in 1935 would be the equivalent of a $100 bet today as measured against the power to buy bread.

I know that doesn't help, but the point is you need to choose your index first.