I stay at Exotic Caye Beach Resort (www.belizeisfun.com
). I book through Kat, aka Tuki Tuki. You can email her at email@example.com. She rents out her one bedroom condo and the price is within the range you are looking for. It is about a 10 minute walk into town, has it's own beach bar, Crazy Canucks; Monkey Business is on site which is a restaurant; Peperonis is across the street which has the best pizza on the island; the grocery store is right around the corner; the resort has an above ground pool and the beach is large and very nice.
Don and Carol the managers are very nice and accommodating.
As for splitting your time, Caye Caulker is very laid back, more so than AC. It is alot of fun and when you go, you have to make your way down to the split and have a couple of cocktails at th Lazy Lizard. There isn't as much night life on Caye Caulker as there is on AC. I would suggest you take a day trip over to Caye Caulker or maybe even an overnighter but it all depends on how you want to spend your time.
Regardless of where you choose to stay, you'll have an awesome time!