$45,000 per year is the economic cost of the time he contributes to the new business.
The difference between the accounting cost and the implicit cost refers to the economic cost. Implicit cost refers to the opportunity cost that the person incurs when he makes a choice. For example consider Geetha is spending something for watching a movie. The cost that she spends for the movie and the cost that can be forgone by her when she spends that for some other things will be included in the economic cost.
In the example given Jim was earning d $70,000 per year and now he is paying himself $25,000 per year for building a new business. Thus the economic cost will be $70,000 -$25,000 = $45,000 per year. Here the accounting cost is $70,000 and the implicit cost is $25,000.
