What do these numbers mean? The likelihood of him going to the Cubs is 50%? What does +500 mean?
This shows how much you win if you bet $100 and get that outcome. So if you bet $100 for LAD to land Harper, and he signs with LAD, you walk away with $600 (your original hundjie back and $500 from the bookie). If you bet $100 on LAA to land him, and he signs with LAA, you walk with $1300.
While there's usually at least some correlation between probability of outcome and payout (the higher the probability, the lower the payout), bookies will adjust the odds on an ongoing basis, to spread bettor money around and protect themselves from having to pay out more than they take in. So Bovada paying out less if Harper signs with NYY than with BOS doesn't always mean they think he's truly more likely to go play for NYY; they just may have a lot of people betting that way and they want to sway some bettors into betting a different way.