...but once you get up into the $50+ range there start to be much greater requirements, yet there is still no standard baseline for the fee structures.
A Coyle shop at $75 is WAY more than twice the work of the Merc shop at $35, but roughly twice the work of an LRA shop that pays $65. Buckalew shops are slightly less work and generally pay $50. CI hotels pay even less but are more work than LRA. There is a lot of variation on workload for a few companies that have paid me $100 per hotel, and even between different hotels for the same MSC, all offering an identical fee.
The one other factor involved in pricing seems to be the "wow" factor of the hotel. Guest Check assignments that pay $200 are generally even more work than Coyle, but they offer some awesome properties. The places I've shopped that generally pay above that rate are less work, but also less attractive properties. If you want to put me in a run-down Sheraton with bad food and make me write 40 pages, it better pay $400-500.
The most I've ever received for a hotel assignment is $600, and that was for an amazing property, but it was also probably the hardest hotel narrative I've ever written. It was also in Vegas...so I was able to negotiate that fee after the MSC started by offering $200 and I knew they didn't have another shopper with my experience and a NV work card available.