best + cheapest device to digitally record audio?

The Griffin iMic sounds like exactly what you need. If you require even higher quality however, you are left with Firewire solutions, such as the ones from M-Audio.

A used digital camcorder might not be all that expensive, and the sound recording capabilities of some (most?) of them are quite good given the size of the device.

if you going to pay for a camcorder you might as get a portable DAT.

http://www.zzounds.com/cat--Portable-DAT-Recorders--2818

I would recommend the iriver ihp-120 together with an external mic. It can record to uncompressed wav or mp3, and you'll have no problems getting the files onto a mac. I would avoid the minidisc - even though the new Hi-MD portable players will let you record to an uncompressed digital format (instead of being stuck with atrac), you'll never be able to get those files onto a mac (the solution is far from ideal even on a pc). There is an exhaustive annotated list covering field recording devices at Vermont Folk Life Center.