What is the most efficient (in terms of cost) way to transfer 35 mm slides to my iMac hard drive?

This gives you the basics

http://www.tech-faq.com/how-to-copy-slides-to-disk.htm

The first option, the professional one, isn't the cheapest - but the results should be good.

The second option is very appealing. Some printers come with brackets that can be used to scan film or slides.

The third option is a slide scanner. I haven't used this option, but there's a significant startup cost of under $100.

In short, a scanner is the cheapest and most efficient option.

I have scanned more than 18,000 slides and negatives using a cheap Epson flatbed scanner with excellent results, but don't overlook the scanner software. I use VueScan.