Are guest hosts paid for being on Saturday Night Live (SNL)?

Yes:

Curious About How Much SNL Hosts Get Paid?

So are we! Fortunately, frequent guest-host Justin Timberlake has loose lips. "It’s the best $5,000 you can make!", he told Entertainment Tonight, before adding "If I wouldn’t have been lucky enough to have the profession I have, I probably would have gone through Upright Citizens Brigade and tried to become a cast member on SNL."

WRT guest spots:

Alec Baldwin revealed that he gets paid just $1,400 for each appearance he makes as President-elect Donald Trump on "Saturday Night Live."

According to an interview Justin Timberlake gave to Entertainment Tonight, yes.

This was my third time to host and obviously I've done some appearances here and there when I'm in town... I really do adore the cast and the writers and everyone there. I mean it really is... there's no show on television in the world, not just America, that I can think of that is like SNL where you can really throw the kitchen sink, as far as being a performer, at people. And it's... you know... it's a great opportunity for an entertainer like myself to.. it's the best minimum... It's the best five grand you can make on television. It's awesome.

Wikipedia lists the base pay for cast members as $5,000 so it would seem guest hosts are paid as cast members.

By the late 1990s, new cast members received a salary between $5,000 and $5,500.