For my son's first Halloween, I dressed him as Mr. Fredrickson from the movie UP. He was 9 months old, and cruising around the furniture like a pro, but unable to walk on his own without holding onto something. This costume provided him a walker, which he loved - and what's cuter than dressing a baby like an old man???
In total, this project cost me about $25, and a lot of the things I bought, like the clothes and stuffed dog, can be re-used. But your costs will vary, depending on what you have around the house to use (duct tape, clothes, stuffed animals, etc.).
NOTE: I've tried to be very detailed, so I've provided a lot of explanation of what I did and why. But you may not need all that, so I put the basic, down-and-dirty steps in bold italics throughout the instructable for easy reference.
Step 1: Building the walker: buy and cut the pieces
First, decide what schedule and diameter PVC pipe you want. I got Schedule 40, 3/4" pipe. Sch. 40 is plenty thick enough for most home and hobby projects, and 3/4" seemed sturdy enough and large enough for the baby to grab a hold of, but not so big that he would lose his grip. I found a 10-ft long pipe at Lowes for a little over $2.
While you are at the store, pick up some connectors. I used two 90-degree side-outlet elbows, and two 45-degree slip elbows (pictured), but you decide how you would like the walker to look/be connected, and get the appropriate pieces. The connectors cost me a total of about $2.
Now you want to cut the length of PVC into the pieces that you desire. If you have a table saw or a chop saw it is incredibly easy. If not, you can use a PVC cutter or just a hacksaw. I'm offering the lengths that I cut for my son, but please adjust for the height of your child (see the next step for how the pieces fit together in the final product so you know which ones to change). For reference, my son was 9 months old and about 28.5" tall. I cut 7 pieces, as labeled in the picture (don't worry about the duct tape or tennis balls yet; I just didn't get a picture of the pieces right after I cut them).
NOTE: If you cut the pieces the lengths I did, you will notice that the whole walker tips back a bit. My son was still putting a bit of his weight on the things he held onto when he was standing, so I compensated for that by tipping the walker back (and by adding weight to it, later). If you have an older child who stands on his own, you may not need to do this - in which case, you may be able to get two 90-degree slip elbows instead of 45-degree slip elbows, and make the front and back legs the same length. Because in my version the back legs (the 11.5" pieces) don't stand perpendicular to the ground, they are cut at an angle so that they lie flush against the ground. I did this by eye-balling them and marking where they needed to be cut after I put the walker together.