Expander cycle engines

Expander cycle rocket engines would be a nice addition, since they could fill the gap of a high efficiency, low power, low mass rocket engine. the full flow staged cycle engines are really efficient, but they are amazingly heavy and expensive, thus making them a less viable option for things like small space probes, reducing their power output is just a half-assed solution, since you'll stil have to carry the weight of the engine. Expander cycle engines could fill that niche. and i think a good set of pros and cons for those engines would be:
PROS: high ISP, low mass, small size
CONS: low thrust, limited size, higher than avegare price-tag.