For low budget diving, Roatan.
You will pay a lot more for flights than Bonaire but everything else is cheap. We thought food on Bonaire was among the highest we've ever seen - both in markets and restaurants. It's much lower on Curacao since it's the deep water port so everything for Bonare goes thru there first.
Stay in the West End, you won't need a car. There's at least a dozen dive shops to choose from, about 30 restaurants ranging from street food to the nicest on Roatan, A small apt with a small kitchen is easily in the $50-60/nt. range. Even less there's two hostels - Chilies is owned by the same family that owns Native Sons Diving. Everybody offers 10 boat dives for $350 - some still $300. A cab ride there from the airport is $5 and should you want to go elsewhere - I used about 6 cabs during the week to goto locations east or south to Cocen Hole - that was the most expensive at $15. There's two small markets in town, Woody's which is like a big 7-11 and the Coconut Tree makret which is smaller. Also the food trucks line one street daily, they sell produce and even fresh fish - it's all on ice and looked decent. We bought fruit from several of them.
I rented a 3br house on the water for $1500 - split 3 ways my total cost for everything including a week of diving, eating at some better places plus we did the dolphin dive at AKR nearby - $1700. I could have done i tfor 1/2 of that. One of our group rented a cabin at Coconut Tree - full kitchen, dive shpp on site, food store on-site, restaurant upstairs. for $45/nt Not all their cabins have kithcens but most at Posada Los Arcos a block away do. It's fronted by the Argentinan Grille and they also make the sandwiches for Sundowners Beach Bar across the street.
Except for flight costs - mine was $785 - it's the cheapest dive week I've ever done. Lots of bugs though.They recommend the Malaria treatement and that will set you back about $200 - look for a travel nurse in your area - cheaper than a travel clinic.