There are several things driving the split grip phenomenon and cost isn't one of them. The additional assembly time far outweighs the any savings, if any, in the cost of the cork. A set of super grade corks for a split grip can cost as much, if not more than a standard grade conventional grip.
1. Fad......they see the pros using them. They want one too........
2. Artistic.....it makes a great place for "rod art" on a custom
3. Balance...... rods are butt heavy by design. Any weight removed from the butt is beneficial to performance
4. Weight reduction.....The lighter the rod, especially in the tip, the more responsive the rod. Reducing the overall weight of a rod is paramount to it's performance and the handle assembly can make up to 60% of the total weight of the rod. Obviously not a big factor if your dead sticking, but it becomes a big issue if your finesse fishing or casting carbon fiber rods in a tournament all afternoon.
5. Customization.......split grips allow for longer, customizable handle lengths at reduced weights. My preference, and a lot of my customer's, is to have the butt land on their elbow. This results in a 15-16" butt which is not readily available. You have to build one up from scratch. Not to mention the increased weight.
Without looking in my records, my customers prefer split grips 4:1 anymore. Now if I could get get them to go acid/ spiral wraps