Not sure how much you'll save per $?

You can get an 8/9 speed Free Hub rear wheel and add a 4.5mm spacer and use a 7 speed cassette.
The cassette can be had for around $20-25. The good part is you have a much stronger rear hub in regards to bending axles. You also have a little bit better selection of gear options, if you don't like the 14-34.

http://sheldonbrown.com/free-k7.html

http://www.jensonusa.com/Cassette-Spacer


The most cost effective way to save a lot of weight is by going to a skinnier tire.