From what I can figure out there is a rule in the appendix about gathering 150 essence to either create or increase the level of a locus. However, there is also a ritual (Drawing down the Shadow) that will achieve the creation much much cheaper. Is there something similar for increasing the level?
There is not a rite for increasing the level of a locus.
As you stated, the only RAW way to increase the level of a locus is to allow 150 essence to accumulate at the site of the locus, and then having it settle over the course of a week. The 150 essence can be provided either by an external source, or by preventing the essence the locus generates from being consumed.