The tiagra-level compact crankset (R-4550?) is pretty darn cheap on ebay and comes with chainrings that are designed for 9-speed set-ups (though I don't think that makes much difference). I run an R-700 with 9-speed 105 parts and it's been great so far.
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"The older you do get, the more rules they're gonna try to get you to follow. You just gotta keep livin', man, L-I-V-I-N." - Wooderson
'14 carbon Synapse - '12 CAAD 10 5 - '99 Gary Fisher Big Sur