Mayweather -667, which means you get $1.15 for every $1 you bet.
No dick you don't get $1.15 for every $1 you bet if Mayweather is -667.
I'll help you with the math, as I see you are struggling.
If you have $1 and decide to place a bet for Mayweather to beat McGregor, at odds of -667 (equivalent to 1.15 in decimal odds).
If Mayweather beats mcgregor, you will get $1.15 (1$ was your original stake, and $0.15 is your profit, and 1 + 0.15 = 1.15)
You are welcome

Fair enough. Why would you chose a lower payout over a higher one?
What's the edge that wagerr provides for you or anyone else to bet on their site and earn significantly less than on any other trusted bookie with higher odds (and no obscure fees, and clear stake limits, and friendly user interface, and so on)?