Well the CPA and CPC for agency services is often that high, because there is a lot of money behind it. So in the end the advertising spend is worth it… but that might not be the case for every agency service out there.
Other good methods are free webinars to get users into your sales funnel.
Content marketing targeting user problems works quite well, as well as seo. If you narrow down your service to 1 or 2 specific things or problems you can easily get a decent seo ranking. Of course if you are a marketing agency and want to rank on no.1 for “marketing agency” than its going to be a though journey ;).
you may also try out partner marketing. Basically find other agencys that are specialized and might want to referr your service to their clients, when they need it. Not a realiable source of leads but it can work.