Take a look at this one ... use the drop menu on the watt section and you'll see that the 400 watter is just $5 more than the 250 watter!

http://www.mdhydro.com/ht-high-pressure-sodium.html

The most economical I have ever found is this "Econo Version" from Inside Sun for $75. (scroll down a little)

http://www.insidesun.com/

hope this helps,
Max Blast