Step 1: What you will need (and where to get it):
If you just want a pre-programmed chip, you can contact me through this website, I will try to send you a chip for $5.50 shipped. Edit: no more pre-programmed chips, also moved to PIC18 for better <3.3V compatibility.
1. The Staples Easy Button (R). Buy it online from here: http://www.staples.com/office/supplies/StaplesProductDisplay?storeId=10001&langId=-1&catalogIdentifier=2&partNumber=606396
or get it at any Staples(R) store for $5.49. Apparently their profit goes to charity as well, so this is good all around.
2. A perfboard or PCB. I have laid out a PCB for you which you can have manufactured here for cheap: http://dorkbotpdx.org/wiki/pcb_order . As of writing a set of 3 boards cost $8.65 ($2.89 ea). The files (in Eagle CAD) format can be found here:https://netfiles.uiuc.edu/zcheng1/www/liljon.zip . Alternatively, get a perfboard (e.g. http://www.radioshack.com/product/index.jsp?productId=2102862 ) and wire it up yourself according to the following schematics.
3. The microcontroller itself (U1). I used a dsPIC33FJ128GP202 . You can also use the dsPIC33FJGP802, or any of the others which have 128KB of flash (to store samples) and come in a 28-pin PDIP (that is, a non surface mount) package. Buy it here: http://www.microchipdirect.com/ (DSPIC33FJ128GP202-I/SP) for $4.93 or sample it for free here http://www.microchip.com/samples/ . Remember to get the SPDIP version.
4. Any resistor from 1K - 4.7K Ohm (R1). This serves to pull up the reset pin (~MCLR) for the microcontroller. This is optional, especially if you have a pre-programmed chip. You can just solder a jumper then to where the resistor should be (R1). This should be about $0.05 if you have to buy it.
5. A NPN transistor (U2). Any common NPN bi-polar transistor will do here. This serves as an amplifier (of sorts, even though it will be operating as a switch and not in the linear range). If you have no idea, a 2N3904 or 2N2222 will work fine. Similarly this should be about $0.05-$0.10.
6. A 10+ uF capacitor (C1). This is necessary because the microcontroller has an internal voltage regulator which is quite noisy. You are however in luck as the original Easy Button (R) has a capacitor which is a 47uF 10V electrolytic, perfect to cannibalize for this purpose. Otherwise get it for a nickel.
7. A board mount pushbutton (S1). This is the most common type of 4 pin push button switch. Get it at Sparkfun for $0.35 http://www.sparkfun.com/products/97 if you don't have a couple lying around.
Your total parts cost is $13.86 (including some money to charity but not including shipping)