I must be much much much older than most of you. The air fares you all are talking about seem very reasonable to me. Air fares are so cheap these days. Don't get me wrong -- I love a bargain but sometimes I cut off my nose to spite my face. Remember -- you are going to Paradise. What's it worth to you?
Do not lock yourself into a certain day or time. I too especially like American Airline and use their web page all the time. Sign up as an Advantage member and they will even Email you when they have specials. (costs nothing to sign up)
How I find the BEST fare (rather than cheapest) is to pick my ideal dates then work around them. I never pick the button "check by price." By using "check by schedule" you get more options.
They also have "multi city" schedules. I'm lucky in that there are 3 (could be 4) airports to choose from (OAK, SFO, SJO).
I can leave from Oakland, but never come back into Oakland as there is a big waster of time. I could got out of SF but would have to leave 2 hours earlier because of commute traffic. I've found it best to fly out of OAK at 6:00 AM via DFW and I'm in Belize City at 2:30'ish.
I regularly find RT for less than $550
It takes a little work, but I find it a lot of fun.
WARNING: Verify everything before you hit the "buy" button anyplace. AA lets you hold a reservation for a couple of days before you have to buy. Once you do you are locked in and it will cost you to make any changes -- like deciding to stay longer.
AmbergrisCaye.com 
