sticking to only what i know, the palms is really nice and you will like it there. nice beach, nice views, nice staff, romantic pool.
the only ruins trip i know of with monkeys is Lamanai - but it isnt cheap. i wanna say like $120 a person, but i think its worth it. a real Belize adventure. i recommend Tanisha tours (ad on right side of this page). no guarantee on monkey sightings, but we saw howlers and a spider.
we're always are on a budget too and save tons by eating half our meals in. the palms has a full kitchen - take advantage of it. have buns & breads from the bakery with coffee & jam from the market and locals fruits from the stand for breakfasts and snacks- yummy and healthy for a only a few dollars. there are lots of good small stands around town with inexpensive local foods from tacos to bbq chicken platters, complete with sides.
there are all kinds of fun things to do there that cost nearly nothing. staring endlessly at the reef is a real draw for me. talk to locals - everyone is happy to talk to you, walk up the beach to town, get up early for sunrise at least once, watch the kids swim at the cut, spend hour at estel's over a cup of coffee and a fry jack or a beer & burrito, snorkel off Ramon's dock (right next door)...
the rest i'll leave for those who know about that stuff. you'll have a great time!