Aruba isn't much like Bonaire, underwater or topside. There's some shore diving available on Aruba but it's not a shore diving haven like Bonaire and doesn't have "Bonaire style" dive resorts. You hear more about boat diving there, and not as much of that either. It does have more wrecks to dive. Above water Aruba has lots more people, shopping, big hotels, restaurants, and lots of American tourists, hence the favorable flights.
I would second the comment about Curacao - the diving has more similarities to Bonaire, though the island and diving are different in their own way. Air service to Curacao isn't as cheap and easy as Aruba, but usually better than Bonaire. Or as mentioned, you can look into inter-island flights, though that can come with it's own headaches and eat up your savings from a cheaper flight.
(If you look at a bathymetric map of the South American coast, you'll see that Aruba lies in shallower water on the continental shelf, while Curacao and Bonaire are near the edge or off, which makes them very different UW than you would expect from just looking at a regular map.)