azure cosmosdb very expensive

You created 18 collections, meaning you have 18 billable database constructs. And with the lowest-cost collection starting at roughly 20 Euro per month (about 5 Euro per 100 RU per month, minimum 400 RU), the math works out: You basically created an environment with 400*18=7200 RU. Perhaps just use fewer collections to reduce your cost footprint (note: collections have no rules around documents being homogeneous). You could use a single collection, since you have such a low amount of data.

EDIT FEB 2018 - note: with database-level RUs (a feature added a few months after the OPs question was posted), the cost model would be very different. 18 collections would be able to share an 1800RU database-level allocation (the minimum is 400RU, up to 4 collections, then 100RU per additional collection). With database-level RU, the cost of an 18-collection configuration would start at 25% of the original cost, based on 400RU per collection.

The problem is that you are billed by collection. So even if you use only a little storage space and you make only a couple of requests, you have to pay the minimum RU per document collection.