Last year I used one of Radio Shack's 20-Second Recording Module kits to add sounds to one of my props. I used the instructions from EarthSands Halloween.
It worked fine but I spent about $20 for materials and it took quite a bit of time to put it together. One downside is there's only 20 seconds of recording time.
In a post last fall on HalloweenForum.com, Zombie-F recommended using Scarey Terry's plans to build a digital sound recorder. Being pressed for time (and a little lazy right now too), I took the easy way out and ordered a couple of assembled units from Cowlacious Designs. For $24 each, I have units that can record 60 seconds at a 8kHz sample rate. I'll be using them with my air props this year.