I've been looking at LED substitutes for my halogens, and
apparently the best technology right now
http://www.leds.de/product_info.php?info=p207_HighPower-3-Watt-LED-Spot-GU10-white--230V.html
can do a 3 W substitute for a 20 W halogen (I tend to use
50 W halogens, though, and 300 W for floodlights) for about 25 EUR.
The field is in a rapid flux, though (given how much has
happened within a year), so this technology should be affordable
in another year and/or new-generation devices available
with more lumen/W, and a better spectrum (I' skipping warm-white
for time being).
