I wouldn't worry about it. It's going to last for several years at least. In the meantime, maybe you can start looking on Craigslist or freecycle for some scrap untreated lumber to get for free!

As Alex said, you probably needn't worry about it. We installed cheap pine four years ago and it's still going strong, even with the humid Nebraska summers.

Don't forget that in a raised bed garden the growing medium dries out faster than a standard garden. Less opportunity for moisture to cause rot.

I used pressure treated 2x6s to build mine. I read the old book last year and got the 2006 edition for Christmas. I know it says not to use pressure treated wood, but my brother's had great luck with it for a decade. I figure I'm fine with it. It won't rot for 20 years and it's sturdy at 3 stories high.

Yeah, I know, I'm totally blowing Mel's system, but my brother's the gardener with a decade of experience and he designed it for me. It'll work and it looks incredible!

Good point about the less rot with a raised bed and well draining soil.

Sinfonian

Here is a link that might be useful: Sinfonian's Garden