That is a very nice looking bed, if money were no problem, I would be filling my lawn with these. I built 2, 4x8 12" beds for roughly $70 (not including filling them) so with just a little work you could get much more bed for less.

I used 8Ft deck board, not the high grade stuff, the low grade, and it was pressure treated. The new supposedly safer stuff. 6 deck boards make one bed, so I bought 12 total. I also purchased 2, 8ft 4x4's also pressure treated.

Cut the 4x4's into 4, 1Ft sections, then cut the remaining peice of 4x4 in half. The 1 Ft sections are used for the corner posts of the bed, with the deck boards being screwed (use coated deck screws) into the 4x4's then in the middle of the 8Ft sections, screw the other longer 4x4's wityh the top being flush with the top of the bed. This way, your center supports are dug a foot or so into the ground, under the bed.

We stain/sealed our beds also, just to help them last longer, and they look nicer. Used american chestnut for the color. For an interesting read, google american chestnut. You will probably be surprised by what you learn.

If you are not in a hurry and live near a Home Depot, keep checking the cull bin in the lumber section near the saw. Most cull pieces are 48" and cost 50 cents to a dollar. I have built nice looking synthetic lumber and also cedar square foot beds this way.