It might not be worth it for you, but it's definitely not greedy to charge that much. Always remember that your freelance rate should be double your full time rate (maybe more) so really it's like a $75/h job or $135k/y equivalent, and then there is the fragmentation of time mentioned, and the fact you have to mentor in real time (unlike coding you can wake up at 4am and get a bit done).

Therefore I think it's a reasonable price to charge. Whether it is worth $150/h to you (or your employer) I guess depends on the business. But if your company has say never used mongodb before (for example), it's not much to pay to get someone with all the battle experience to come in and get people up to speed rather than say have production outages as people didn't know what they were doing, or alternatively hiring someone full time at a cost of $200k just for their mongo expertise.