All those rates sound very low, especially when you look at what you actually net after taxes and all that.
$75/hr would seem the be the floor, IMO, unless you're in a very low-tech area like the middle of Utah or something.
Having been a prime contractor who hired subs for overflow work, we'd generally never go above 2/3 of the prime rate.
If you want to make $100/hr, you generally have to go sell yourself to a customer, not act as a subcontractor.
Someone who is already well known and respected for what they do, you should be prepared to offer a better deal for. In fact, you may end up being able to raise your own outgoing rate if you find folks willing to work for you who are really great.
But of course, the risk does end up in the hands of who owns the contract. And if there isn't sufficient trust with between you and your subs, a premium helps protect you from that.
But this practice may be keeping you locked into a cycle of only getting help from low to mid skill level developers, who end up going elsewhere once they've grown a bit more.
Seeing people discuss rates in the SV area just blows me away sometimes.