We have several dive centers to choose from.
As for the price they are all with in a few dollars of each other.
If you are going to do several days of diving most will give you a discount of 10%
Try look at Belize Diving Adventures (owned by a couple Everett and Inez both instructors and they take out small groups)
Chuck and Robbies ( no frills but they are know as one of the best here)
Belize Island Divers ( located in the middle of town, has a great following with the local people who dive)
All three will pick you up from your hotel and will give you a excellent service.
As for inexpensive stays, look at Ruby's, Pedro's inn, or the Conch shell. All are very reasonable