There are a couple factors that make pager fees higher on the weekends.
One, people generally value weekend hours more than weeknight, so it's not completely crazy.
Two, cardiac pager call generally has really low call-in rate. It's not like general call or god forbid OB which often amounts to an expected overnight in-hospital shift. $200 for holding the pager and $250/hr with a 2 hr minimum for call backs is quite reasonable IMO for rare call-ins.
Three, locums people who book a week or a two at a time are going to be present during the week, probably doing nothing but hanging out in a hotel room. A smaller fee, with a near-zero call-in rate, when you're staring at the wall in a hotel room anyway, is where that market naturally settles. But weekends are different. Before/after a week's work, a locums might not even want to be in town at all. It's common to show up Sunday night to start working Monday, or to GTFO of town the minute that work ends on Friday. If you want a locums to spend the weekend in your town, you need to pay more money.
I think the reasonableness of those call rates really depends on how often you get called in on those weeknights and weekends. The $250/h callback rate is solid, provided there's a minimum hour count for any callback. If they call you in and then decide not to operate after all, they should owe you more than a 15 minute increment of the hourly rate.