Actually the LEDs are not even half of the cost! http://www.dealextreme.com/details.dx/sku.2395
is the cheapest source I have found for high power LEDs suitable for our use. Note that this particular batch is a "bin" of lower lumen output high power Cree LEDs.
Just a flat aluminum plate might work ok, but keeping the LEDs cool is a major requirement for long life and most output, and they do get very hot if not actively cooled. A finned aluminum heat sink extrusion is obviously the best idea, but my local surplus metals dealer sells aluminum extrusions, not heat sink extrusions, that include all sizes of channels. They are sold in 8 foot lengths. So, for a 20 inch long heat sink, that gives you 5 lengths. A 10 gallon tank is about 10 inches in depth, so 2 inch channel would give you an almost perfect fit with the top. But, you don't need that, so a 1.5 inch channel gives you a 7.5 inch wide heatsink, which is perfect. And, 1.5 inch channel is only about $15 per length - very cheap. But, you have to use a heat conducting grease between the channels, and rivet or screw them together, side by side. So, $15 plus about $5-$10 worth of grease, and another $5 worth of nuts and bolts will give you a heat sink.
The DC power supply has to provide enough amperes to drive all of the LEDs, and at a voltage that lets you put the LEDs all in series, or in parallel groups of equal numbers of LEDs in series. Each LED will use about 3.1 to 3.6 volts. If 16 are put in series, the DC voltage required is 16 x 3.6 volts, or 57.6 volts, but you need a bit more in order to limit the current, so a 60 volt source is needed. The LEDs, for maximum output, will need .7 amps, so that 60 volt supply has to be capable of 700 mAmps output, preferably up to 1000 mAmps (1 amp). It is hard to find such a AC to DC converter with those parameters.
To control the current you need 1% resistors, maybe at worst 5% resistors, able to handle the power you will drop on the resistors.
For best cooling you need two fans, 12 VDC and a power supply for them, or 110VAC, which are more expensive.
To mount the LEDs you need a heat transmitting grease or adhesive between each LED and the heat sink, and that stuff can cost from $3 to $20.
Even hookup wire isn't free, and nuts and bolts, switches, etc. all add up too. And, you still don't have a nice looking housing for the light fixture.
The important thing is to really enjoy the adventure of learning about LEDs, electric circuits, designing the fixture, and making it.