So I had this idea...LED rope lights...they come in cool and warm white and for larger tanks you could mix both.
So here is the basic idea... These things sell pretty cheap on ebay even in larger sizes especially compared to getting similar numbers of these type leds in other fixtures... Find the biggest rope light you can that you think curled up will fit under a glass top...silicone it into place, cut notches in back plastic strip for cords and ta da...cheap led lighting. You could even spray paint the glass top white on one side (side for the lights, and then black (if you wanted to) on the top to make it look nice). Using flat white paint should reflect around 85% of the light, most back down to the tank floor. (Thats what flat white generally reflects, I didn't make up that number)
You should be able to get a curled up rope 6ft rope light to fit under a 10gal glass top, if not quite then I believe the rope lights can usually be cut shorter leaving the rest of the LEDs working. Especially with a white top, given the number of leds and total wattage of the rope light this should be adequate if not excessive for 10-20gal long. You can find rope lights in a bunch of lengths also for longer tanks...I think this will work for other size tanks that aren't excessively tall also but you'll probably need the white painted top for sure there. I'd experiment though before you paint the glass top of a 30 gal or bigger tank...maybe use cardboard or a piece of wood painted white to test the idea and make sure its enough light. (in fact this could be your top if you wanted)
Another solution that could be used alone or with rope lights are these (which probably isn't a new idea, but thought it was worth mentioning again)...
225 LED AQUARIUM GROW LIGHT PANEL HYDROPONIC WHITE 110V - eBay (item 180555882180 end time Sep-08-10 19:05:20 PDT)
Also someone linked to these, and I have seen other similar fixtures elsewhere...
IKEA | Integrated lighting | Bookcase integrated lighting | DIODER | Lighting strip
A set like that could probably light a 10gal or 20L all by itself...
I'd wager the right combination of these lighting solutions could even do a 75gal or larger. Really tall tanks are the ones they will probably be less likely to work well on. An array of LED spot lights possibly in conjunction with the lights mentioned here might be better. Using a fixture with downward pointing sockets like this...

(this has a tube light also, other manufactures make them without the tube)
Something like that in conjunction with the robe and under-cabinet lighting would probably work on a taller tank for cheaper then most aquarium led fixtures.
Just a few ideas, hope they are helpful to someone. Not all will allow for the white painted top but you might be able to partially mask off the top and allow clear strips for other lights if you use a combination. I'll likely try the under-cabinet led and robe led on some of my smaller tanks eventually.
You should be able to do a 10gal for under 30 dollars or close to it, thats including, glass top, the rope light, paint and silicone.