DIY platform bed with approximately $60-$90 that definitely will last. Made with plywood, 2x6's, 4x4's, and castors (wheels). More pictures available at www.myplatformbed.com. You will need:

Materials Needed:
3 - 2x6 Boards 10' feet long
1 - 4x4 Post 32" inches to 40" inches long
1- 2x4 Stud
12 - Hex Bolts 3/8 by 6" inches long
12 - Hex 3/8 Nuts
24 - Cut Washers for 3/8 Bolts
1 - 2x4 Stud framing support bracket (pictured)
8 - Screws; 1 1/2" inches

Tools Needed:
Carpenter Square
Tape Measure
Miter Saw (Recommended)
Electric Drill with Phillips Drill Bit
Long 3/8 Drill bit (6" inches or over)
1- 9/16 Socket Wrench
1- 9/16 Wrench
Wood Glue

Step 1: Making Legs

We start by cutting the legs of the platform bed. This 4x4 wood I found outside, discarded. Find a scrap one or buy, cut yourself or cut to size at the lumber store.

Cut 8" in length if you want castors on it, 10" if you do not. You may opt for shorter legs (no shorter than 6.5") or longer ones.

Step 2: Cut Your Head/Foot and Side Boards

With my cut I am cutting one inch bigger to play it safe. I am fine with a platform bed approximately 1 inch bigger than the mattress, if you prefer the platform structure even out with your mattress, cut exactly as your mattress is. With your 2x6 board cut:

Mattress Size: Long Side Board Cut: Short Head/Foot Board Cut:
Twin two 75" boards (or 76") two 39" boards (or 40")
Full two 75" boards (or 76") two 54" boards (or 55")
Queen two 60" boards (or 61") two 80" boards (or 81")
King two 76"/78" boards (Kings varies) two 80" boards or (81")
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My particular platform bed is Full Size. There's a popular quote in carpentry that goes as "Measure twice, Cut once". Be careful with your measurements. From this point on, there will be more measurements needed and I recommend a Carpenter's Square for straight and even lines to cut from ($2.99 yellow plastic tool pictured above)

Step 3: Attach Short Boards to Legs

The entire platform bed will have a 1/2" gap from the top, nothing will obstruct that 1/2" inch gap all around the platform bed. This is done in advance to secure support beams/boards (plywood or wood strips) at the end of the project.

For your short (head/foot) side boards you want the edge of the legs and board to even out, as shown in picture above. Remember to maintain a 1/2" inch gap. See previous picture #2.

Step 4: Attaching Long Side to Bed

We apply the same 1/2" inch gap from the above and will be drilling for two bolts on each ends. This time the edges of the leg and board will not even up. It'll be off by 1 1/2" inches (thickness of my 2x6).

Next, make measurements to locate where the first hole is on the leg and draw an "avoid line" on the board, just for reference. If you measure from the top of the leg, remember to account for the 1/2" gap. Draw a line 3" inches from the edge because you will be drilling 3" away from the edge into center of the leg.

With your "avoid line" and by the 3" inch mark, drill a hole and insert a bolt between 1 1/4" to 1 1/2"inches from the top and another one 1 1/3" to 2" inches from the bottom.

Step 5: Install Center Support Beam

Installing the center support beam for the platform bed, begin by marking the very center of both sides (head/foot side) of this bed. Also mark for our 1/2" inch gap to find where the 2x4 support stud beam should go.

Then trace lines around where you want to install the 2x4 Stud support bracket and disassembly your platform bed so you can drill on screws better and hammer in some built in notch on the support bracket.

Do the same for the opposite side, then measure the length from that stud support bracket to the other support bracket on the other side and cut a 2x4 stud of that length. That measurement should be within an inch or two of the length of your mattress size.

Step 6: Glue/Nail on Support Rail

My support rail is long and thin (scrap wood 1/2" thick) cut into two 4' feet strips and pre-drilled (with a small drill bit) in four places ready for screws. I apply glue to one side of the support rail and screw onto the inside of the board with my 1/2" inch gap in mind. Pre-drill your rail and use 1 1/2" inches screws.
Note: If you select a wood strip support system instead of plywood, make your support rail the entire length of the side boards. You can't afford gaps with a wood strip support system.

Step 7: Install Castors, Cut Plywood, Tighten and Enjoy!

It is fairly easy to install castors, just flip over the platform frame and pre-drill your four holes where the castor is centered then screw in small screws around 1" inch long.

You must already know we're almost done here! The last thing to do is get some support for the mattress and you. I am using plywood because they're still light and won't buckle too much as a wood strip support system might. Measure the sideways length of the inside right above on top of your new support rails and cut sheets of plywood of that size. I made two sheets of plywood with their standard width but the third sheet I had to cut to fit.