There are two ways to look at this. One figure out how much it is per hour for a whole year for you to be on call, which figures out to be 1.05 per hour. Or figure out how how much flying it's going to actually be and figure it out that way.
Managing a part 91 Navajo is like managing a Honda Civic. If you are waiting for an airline job and this guy only flies once a week and wants you to take care of his airplane, do stuff like cut grass, instruct or something like that to fill in the time, it's better than getting completely out of aviation and then trying to get back in. Looks better on a resume too. I don't know your financial situation but, you have a base salary of 27,000 and everything else is just icing on the cake.
No offense to pudnocker, but an owner must be a rich crackhead to pay someone 53k to manage a navajo.
The pay still beats a starting regional job.