What stop do you get off at on the Long Island Railroad to go to the Long Island MacArthur Airport (Islip Airport)? How much does it cost from the train station to the airport?

Ronkonkoma station on the main line. Cost by cab varies but is inexpensive compared to long term parking for a few days.

The closest station is Ronkonkoma. During the day there is a Suffolk Transit bus (possibly S-56) which go to and from the airport. Also, taxis are present at the railroad station. The other option is Sayville LIRR station which is further south of the airport. Taxis are there also, 24/7.

Depends where you're coming from because of transfers. The closest train station is Ronkonkoma. I'm not sure of rates of taxis to the airport though. But it is a really short trip. Maybe like 10mins and technically in the same town, despite it being called "Isp" airport, so it shouldn't be too much, since there shouldn't be an extra charge per town crossing fee.