I'd think it could definitely be substantially cheaper than Inmarsat. In a totally crushing way. So much more bandwidth capacity and lower latency.

Note: SpaceX is building out the Starlink coverage in a gradual way - they won't have satellites around the poles early on. Until the have truly global coverage Inmarsat will obviously stilll have an edge.

It will be interesting to see how the pricing plays out for the high latency satellite broadband offerings compared to the low latency Starlink offering.