From airport directly to San Ignacio??

There are no public buses from Philip S. W. Goldson International Airport (PGIA). A taxi to Belize City costs BZ$50.00 (per taxi, not per passenger), and takes 15 to 25 minutes depending on traffic.

The really cheap option is to walk or taxi from the airport to the Northern highway (about 2.5 km) and wait for a bus. As you approach the intersection with the highway, Belize City will be to your right so just wait on the side of the highway. When you see a bus coming, wave your arm up and down to get it to stop; Express buses will not stop. The fare to Belize City is BZ$1 or $2, and the bus will terminate at the bus terminal where you can change to a bus to San Ignacio (BZ$7 to SI).

For more money, here’s the link to a transfer service (I have no experience with this service, but have read some positive comments):
http://www.discountedbelizeshuttlesandtours.com/index.html

William's Belize Shuttle is also a good transfer service, but don't confuse it with another service called Belize Shuttles & Tranfers and has received some not so positive comments.

A transfer service will pick you up at the airport and take you in comfort to SI.

We also did the ATM with Pacz. Our guide was Oscar, and he was terrific. Encouraging over the more difficult bits and also properly respectful of the sacredness of the cave. We did book in advance because we were there in December, a busy time, and they do restrict the number of visitors in the cave, and we wanted to go on a particular day. You can book online with Pacz. We stayed over in Guatemala, and we booked the Tikal tour through the Tikal Inn, but we were picked up in Flores.. Maybe not the cheapest way, but the guide was very good, not only on the archaeology, but on the birds and wildlife too. I think the company was Eco-Adventure (or similar).