Amazon lists Alex aluminum rims starting at under $10 in 700C size. Order several to reduce shipping per rim as due to size they are not cheap to ship.
http://www.amazon.com/Alex-REDLINE%C...951809&sr=1-20
or
http://www.amazon.com/Alex-RP15F-Rim...951809&sr=1-23
Personally I would want a strong and useful wheel when done so suggest a Sun or Alex true double wall construction rim such as the Alex DM18, Sun CR-18 or Sun M-13 rims. These are listed on Amazon in the $19 to $26 range and can produce decent wheels when done. The Sun M-13 is a narrow rim for 28mm or narrower tires and the Sun CR-18 is wider and a good touring or city bike rim.
Use decent components and even a first wheel building attempt can be a good wheel that can provide years of service.
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Gear Hubs Owned: Rohloff disc brake, SRAM iM9 disc brake, SRAM P5 freewheel, Sachs Torpedo 3 speed freewheel, NuVinci CVT, Shimano Alfine SG S-501, Sturmey Archer S5-2 Alloy. Other: 83 Colnago Super Record, Univega Via De Oro
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http://groups.yahoo.com/group/Geared_hub_bikes/