First off, what is a bike tree? As far as I can tell, it's a free-standing structure meant to hold one or more bicycles vertically. It's great for keeping bicycles organized, and generally takes less floor space than leaning them up against something. Also, less chance of nicking your hardwood floor with a kick-stand.
I based this design largely off of one done by zzpza, which you can see here: https://www.instructables.com/id/Bike_Tree . Being the flat broke student that I am, I wanted to spend less than the 35GBP that he spent. I did. My final cost was $3. Granted, I had a bit of wood I scavenged from an old project, but even if you don't have that advantage it shouldn't cost more than $10.
Step 1: The Materials
Here's what you need:
(1) 2'-6" x 2' plywood or otherwise planar surface.
(3) 2' long 2x4s
(2) 5' long 2x4s
(4) 3' long 2x4s (approximate, you'll trim these ones down)
(2) 1' long 2x4s
(2) 6" Bike hooks
(30) 2 1/2" wood screws (you can do it with less if you want, or more if you're crazy)
(3) 1 1/2"x2' strips of heavy fabric
As for locations and prices...
Get the plywood from the scrap pile at Home Depot or somewhere similar. It shouldn't cost more than a dollar. If you're in a craigslist friendly zone, you might try looking for people tossing out desks or other stuff. Maybe you can get it free.
if you're planning on buying brand new 2x4s, I'd suggest modifying the design to use alternate materials. If you've got enough leftover plywood from the base piece, replace some of the angled supports with it. It's be an easy swap.
The 6" bike hooks run about 70 cents or so at Home Depot. Don't buy the expensive ones, the cheap ones work just fine.
Get the fabric from rags, an old shirt, whatever. I had scraps of canvas from another project that I used.
I'm not really sure how to get cheap screws, so just buy a handful from where ever.
Step 2: The Base
Screw the 2' long 2x4's to the 2'x2'-6" plywood. They should be oriented perpendicular to the surface, one on each edge and one in the middle. Three screws per 2x4 should be plenty.
Place a 1' long 2x4 on the opposite the plywood, screw it on flat in the middle of the board, with an edge touching the end.
Flip it all over and glue the fabric on to the bottom of the 2x4s. If you have longer pieces, maybe you'll want to use nails or staples to tack it up on the sides. If it's a heavy enough fabric, it should help keep from scratching your floors if it gets jostled.
I glued mine on with white glue and tacked the front. Sure, it's not very durable, but ya know? I couldn't care less. I'm not planning on moving this thing too much.
Step 3: The Supports
Ok, now you're coming up on the only mildly difficult part of the project. These angles are pretty important, so you'll want to get them right. I've uploaded a CAD image with the specific dimensions, so I won't write them in here.
If have access to a miter box or a chop saw with angles, it'll make your life easier. If not, draw the lines and cut with a circular saw, you'll get the hang of it pretty quick.
Once they're cut out, attach them all to the plywood. You'll probably end up standing it on its side while you screw them in from underneath and on top. The pieces with the 45 deg angle will also screw into the 1' 2x4 on the base.
Make sure you have all of these supports securely attached to the 2x4's beneath them. The plywood is just there to be a surface, not provide strength.
Step 4: The Vertical
Screw the 5' long 2x4's together, side by side.
Attach the vertical to the base and to the supports. The base should be secured dead center in the horizontal direction, and approximately 1 1/2" from the back edge. Use a level if you can to make sure it's totally vertical. While having it off by a bit won't matter that much, it'll look better if you do.
Step 5: The Hanger
For the hanger, take the remaining 1' 2x4. Drill holes in it approximately 3 1/2" off of the center, or however far off to you need to so that both hooks can be horizontal without touching the vertical.
Screw in the hooks. You want to hook portion to be pointing away from the center.
Take the block and mount it on the vertical. The bottom of the hanger should be approximately 4'-5" from the top of the base, though you should play around with your particular bicycle in order to find the optimal height for your particular situation.
Step 6: The End
That's all there is to it. To mount the bikes, lift them up and place the top wheel in the hook. Then roll the back wheel into the corner of the of the supports at approximately a 30 degree angle off of center. If you want, you can add runners to the top of the base so you always put the bicycle in straight, but this isn't necessary.
You'll note that the pictures show a much higher vertical than what I've recommended. I was making it as I went, and wasn't really sure how high to place the support for the bicycles I had, so I left the 2x4's at the 7' length that I found them in. Ignore that fact.
I hope you've gotten some use out of this instructable. As I said at the beginning, the whole premise was to make a second generation of zzpza's design. Thanks go to him for posting his instructable, the Berkeley Public Library for an awesome tool library, Ari for moving tools and materials around in her car, and school for making me poor and leaving me need the cheapest possible options. Also, graphics were made in Google Sketchup. The dimensioned image was done in AutoCAD. Photos were taken with an old cell phone. Ciao!