SQL Server Web Edition is not the same as Standard or Enterprise. The reason you use SQL Server instead of PostgreSQL is for Database Encryption, Analysis Services, AlwaysOn Availability Groups, etc. Web Edition won't even do mirroring.
The cost is worth it if it saves you multiple employees worth of development work or administration overhead, and in my experience it does. The pricing is also (surprise!) negotiable. At Loopt, our annual software licensing cost was well under the cost of one developer, and this was before BizSpark.
And you also get credits from Bizspak, that could be used to apply to offset AWS licensing.