This is kind of a problem if you think about it, the grinding system could have been improved drasticly simply by providing more variety of missions to do and other fun ways to gain money, other than grinding the same one over and over again which makes it boring.
Let's take GTA Online for example, if you want to buy that 4 million dollar plane from the new DLC, you have to grind the pacific heist mission again and again, each grants you 100k, it takes more than 11 hours to get that plane, unless you buy a shark card which is expensive.