Exadata:Hardware used to be competitively priced (at least in the x4 age), its the licensing for the software on top that makes it really expensive.
The other thing is that some oracle features only work on exadata or other engineered systems. The hybrid columnar compression for one and query offload as well as some forms of the in memory stuff. The problem is that these are very high end features and most of us live happily without these. Postgres is going after the bread in the oracle db market while exadata is the exclusive cheese shop. Much smaller market even if the margins are better.
