You may already have all the parts you need to make this simple little practice amp, in which case it would be free!
Step 1: What You Need
Make sure you get the ones that plug into the wall with an adapter. Also, get ones that have a headphone jack built in and have the cable with the plug that connects to the computer (usually green). BTW, USB powered computer speakers won't work.
I already had a set lying around but you can get a similar pair for less than $5. Go to your thrift store or swap meet, don't pay more than a few bucks for a set.
You'll also need a soldering iron, wire cutters and phillips screwdriver
If you're lucky, you can find a set that works off a 9V adapter, in which case you can mod it with a 9V battery for complete portability. If you are even luckier, you'll find a set that already is made to work off batteries.
Step 2: Open the Case and Prepare to Modify
I wanted mine to look like a miniature guitar amp so I threw away the 2nd speaker.
Clip the wires and leave a few inches of the input wire (the one with the green plug)
Step 3: Solder and Drill
Drill a hole for the 1/4" jack. Don't make it too big or the jack won't tighten. Remember, start small - you can always make the hole bigger - it is really tough to make the hole smaller later.
Also, the guitar cable needs to go into the jack, so watch out where you drill the hole. I made it on the opposite side of the headphone jack and a bit towards the rear to make sure the cable plug didn't hit the speaker when I plugged it in.
If you are going to add a 9V battery connection, do so now.
Step 4: Done!
Step 5: Play!
OK, don't make fun of my guitar playing or my vertical iPhone videography, this was a quickie.
The amp is not going to be super loud but it is enough to make some noise and if you plug in your headphones, it is plenty loud.