What are the most expensive services your company pays for?
For example: AWS, Salesforce, etc.
Humans and Rent.
If you mean technology-wise, Office 365, Box.net, Adobe Creative Cloud, and a bunch of back office crap like SAP
Office Space and to avoid the same we've been working out of cafes or piggybacking on the office space of other startups while we can (we hope to keep this culture going forward and encourage other early stage startups to work out of our office space once we establish ourselves).
In order - Humans, OS/DB Licenses, Rent, Hosting Infrastructure, Marketing/Sales, Misc Tooling (like 365/VS/JIRA)
Developers, AWS is very low < 100 month.