Also it's worth noting that 10 watts while not a lot in absolute terms could be the difference between being over threshold or not. The body's ability to produce and sustain power is not linear and when you are at the edge of your threshold any savings you can manage is beneficial. Also it looks like 20mm rims are closer to 30W and 60mm rims are getting down to 15W so you can save even more. As to the cost, that's east enough to figure out. 15W = $CostOf(60mm) - $CostOf(20mm)