Well, I'm no insider, but I've been avidly following DwtS on the boards for years. I'm pretty sure they all sign a multi-contract season -- I've read for 5 years (but it may mean 5 seasons, since we have two per year.)
The show decides, a few months ahead I guess, each season which pros they want for that season. Louis van Amstel posted on his blog that a lot of considerations go into their casting decisions - and he was sorry that he wasn't selected this season. I believe his last season was Priscilla Presley, whenever that was - but pretty recent I think, and it came out later that she requested him specifically (Priscilla's good friends with a former partner of his, Lisa Rinna.) Wayne Newton requested Cheryl Burke - maybe others have had the clout to pick the pro they want.
Having the dancers on contract enables ABC also to request - well, probably, "demand" that they perform on the results show's pro dances whenever ABC wants them to.
I think it's been the show's, not the dancer's, decision every time one hasn't been asked back - like Louis or Nick or Inna or Alec or Brian Fortuna (though he got picked up by Strictly -- plus he did the DwtS summer tour after his only season on Dwts with beauty pageant contestant Shandi Finnesey.)
Maks "claims" he requested time off and got it - and maybe he got away with it because he's like their main hunk - but I don't think the others can take a season off, just because they want to. I think it's up to ABC, as long as they're still on contract.
Years ago, I read the dancers make about $7000 per week - plus an additional payment for every "pro dance" they do ($1500 maybe?) Oh I don't know -- and even if these were correct, it was several years ago. At the time, I think the celebs got $25,000 for every week they stayed in - with some sort of guaranteed minimum - and a $100K bonus for winning ?? Oh those figures may be way off, it was just someone posting on a message board quoting the figures, and they're out of date. But the "ballpark" may be about right.
Oh and I think they paid Kelly Monaco a million dollars to do the "re-match" with John O'Hurley after season 1 because she didn't want to do it (she won the first season but the fans weren't used to the "system" and there were a lot of complaints that she had made a mistake on her perfect-score free-style or some such.)