I made this trek two years ago. After researching this route, the most cost effective was to take a Go Opti shuttle between Ljubljana and Zagreb airport. There are several pick up options in terms of being picked up during a certain time window or being picked up a certain time. The rides are shared, but the cheapest option runs you less than $20 per person. Great deal if you ask me. You save more booking further in advance. And from Zageb airport it's about a $60 flight to Split. The ride from LJU to ZAG is about 2 hours and then ZAG to SPU is a one hour flight. All for less than $100 :-).