Before we start on a design, we spin through a local handyman paradise to get a feel for prices. Listed below are the materials needed and the estimated cost that we came up with in our search (prices vary according to location and store).

TOOLS YOU'LL NEED

Goggles
Plumb line
Dust masks
2-foot level
Reversible drill
Jigsaw
Hammer
Handsaw
Combination square
Tape measure
#2 Phillips head screwdriver
Posthole digger
Steel wheelbarrow (for mixing concrete)
Shovel
Extension ladder

SUPPLIES

Two 40 lb. bags of concrete mix: $4.65
Two 6" lag screws: $1.35
3" galvanized screws, three packs of 20 each
  (for roof, balusters): $6.27
Two pounds 3 1/2" galvanized nails: $2.75
Five pounds 2 1/2" spiral deck nails: $6.85
Plastic tarp for roof: $3.00

Subtotal:  $24.87

LUMBER

Two 4'x 4'x 16'(for posts): $24.54
Seven 2'x 6' x 8' (for frame, joists): $24.64
Fourteen 5/4'x 6'x 8' (for floor): $87.00
Four 2'x 8'x 8' (for railing): $19.80
Twenty-eight 2'x 2'x 8' (for balusters, roof): $53.48
Five 2'x 4'x 16' (for diagonal supports,
  ladder, rungs): $21.50

Approximate Total:  $255.83

TIPS:

1. At the lumber yard, "sight down the wood"--that is, hold one end of the board up to your eye and look down its length for unwanted curves.

2. Inspect the wood for knots, and not just for aesthetics. Knots warp the wood, and they're hard; your nail hates to hit one and so will your arm.

3. For the wood, choose pine; a soft, abundant, inexpensive wood; treated with arsenic--a handy dosing that retards bugs' interest. Otherwise, termites and carpenter bees bore in and lay eggs. Redwood would be ideal, a durable, light wood that bugs find naturally repugnant, or a hardwood like oak, but they are both more expensive.