What hasn't been addressed is pay for right seat time. This comes in two flavors. One, distinctly, and without question, is Gulfstream style pay for a job time. One should never pay for a job in this biz. Doing the Gulfstream program is bad for the profession and bad for the career. There is some well deserved prejudice towards Gulfstreamers out there in the biz. I've seen it mentioned for years on the net. Sadly, I think, some who have PFJed at Gulfstream found it to be a career enhancing move. YMMV. Keylime may fall under this category.
The other flavor of paid right seat time is the EagleJet/Ameriflight. This is where a loop hole in the FAR's is manipulated to allow for a copilot in what would otherwise be a single pilot operation. The copilot can be bumped for freight, so one has to wonder just how important such F/O time can be. It's logable time as it's legal per FAR's. You aren't paying to log time in a seat that would otherwise need to be filled. However, some will see this as useless time, thought it was legal per the FAR's. Keylime may fall under this category.
ATP flight school has a program where two guys log PIC time in a twin. It's another loop hole in the FAR's but a perfectly legal way to build multi time.
If the OP has would like to be more specific, there are folks here that could get more into detail on the question.