Sebastien Bubeck points out COLT registration with a May 13 early registration deadline. The local organizers have done an admirable job of containing costs with a $300 registration fee.
ICML registration is also available, at about an x3 higher cost. My understanding is that this is partly due to the costs of a larger conference being harder to contain, partly due to ICML lasting twice as long with tutorials and workshops, and partly because the conference organizers were a bit over-conservative in various ways.