A horn player I know told me recently that he doesn't think of it as getting paid for playing, rather he considers he gets paid for driving, setting up, soundchecking, rehearsing, waiting around, etc.
Actually I'd pay them the same as everyone else, even if they do just turn up and play and bolt. How are you going to monitor attendance, timesheets? Punchcards?!
I've played sax for about 8 years. Setting up is a two minute process, and it's not hard to carry an 8lb instrument into a venue in one trip. Compare that to drums, multiple guitars, amps, keyboards, PA equipment, etc... They have a much easier job than the 'core' band. They can also have the luxury of showing up 10 minutes before the gig and leaving immediately after. I certainly don't have that ability (and I'm pretty darn quick at my tear down and setup). Plus in my experiences, horn players rarely come to all rehearsals, rarely help book, rarely help network, and rarely bring other talents (be they musical or business based) into groups. Of course there are exceptions, but based on personal experiences you'd have a hard time selling me on equal pay unless you pulled a lot more weight around as a horn player.