2 Different database or 1 Single database
Following up on @JonathanFite's comment, current and future resource usage is critical to answer this question, specifically memory capacity and I/O write performance.

Rule of thumb: if both databases can fit in RAM with room to spare, run both engines. If memory is limited, consider migrating the smaller database of the two, to the engine running the larger one.

Because you're most concerned about hosting costs, it will probably work out cheaper to consolidate, but you may have to increase your memory down the line.