Ingo from ArangoDB here:

If you need to scale-out, the ArangoDB packages are more affordable then MongoDB Atlas, as you don't need to spin-up a whole 3 node replica-set to add another shard to your cluster. The smaller instances are cheaper in Atlas, here you benefit from the established cloud service, which can negotiate better conditions with the large cloud providers. However, we will pass on lower cloud costs to customers, so there is hope that we will move closer over time. But, I don't see ArangoDB in direct competition with smaller, pure document-use cases. Most users need the multi-model capabilities and use graphs in combination with document operations.

(ArangoDB developer here) MongoDB is a document store, ArangoDB is multi-model, so you have graphs and key/value and search as a additional benefits. The query language AQL is also a huge plus. Price comparisons are always hard, but for similar deployments Oasis will be a bit cheaper. One really has to look at the details here, in particular with respect to sharding and resilience.