Amazon Neptune is a fast, reliable, fully managed graph database service that makes it easy to build and run applications that work with highly connected datasets. The core of Amazon Neptune is a purpose-built, high-performance graph database engine optimized for storing billions of relationships and querying the graph with milliseconds latency. Amazon Neptune supports popular graph models Property Graph and W3C's RDF, and their respective query languages Apache TinkerPop Gremlin and SPARQL,...
When taken as part of the whole AWS contract, it was cheaper, but as a single service, Neptune was more expensive compared to Neo4j.
Microsoft Azure and Amazon AWS are on par for pricing and Google has been raising its prices.