Hi Alix,
While I haven't chose Casa Magna for my wedding, i will tell you that it is a beautiful hotel. I am a flight attendant and this is the hotel that we lay over at while in Cancun...let me see if I can dig up some pic, I have been on maternity leave for over a year so it has been a while since I have been there.
As far as pricing goes, you may want to check out the palace resorts, they are very reasonably priced even for private functions. My wedding date is about a month before you, so I am sure that you know it is very hard getting any pricing this far out, but the palace resorts seem pretty consistent with pricing.