The cheapest solution I found was EcoSmart brand LED bulbs at Home Depot. They're all about $40 ea. I got one that is 18 watts (1200 lumens) in the 4900K range. Be sure to check the Kelvin rating because most of these bulbs are in the 2000-3000K range and that won't do. I doubt it will do well for corals, but I have it over my 29G hex refugium and my macro algae has really taken off. I even have some faint purple coralline algae starting to pop up. It runs completely cool and doesn't raise temps in my tank.
If you want corals, I would look at the PAR bulbs from Orphek. Pricier, but in the right ranges for coral. I'm looking into it for my 55 gallon in a similar setup to your own.