Maybe it would better to go somewhere like Utila. Flights are almost triple but diving and food/accomadation would be much cheaper so they end up being somewhat even.
Price Roatan first, Utila costs several hundred more typically since access is somewhat limited to local flights from either Roatan or San Pedro Sula/La Ceiba or expensive boat rides from Roatan's West End. If you read about the Princess Ferry - afaik it's still in drydock.
Stay in the West End - there's two hostels, Chillies or Buena Onda. Food is cheap - from street vendors to decent meals in the $10-15 range. Two chickens at Rotisserie Chicken cost me $22. Noodles Factory was $5 for lunch. Sundowners has sandwiches for $6-8 beachfront. There's two smaller markets in the West End for staples. Larger markets are in Coxen Hole or French Harbour - probably $20 via cab - negotiate... My friends rented a cabin with a kitchen at Coconut Tree and we found everything we needed for breakfast/snacks locally - we ate all other meals out.
Dives are 10/$300 most places - $40 per night dive. Chillies is affiliated with Native Sons - we used Coconut Tree Divers next door. They have a good general West End info page here:
Roatán, West End | Scuba Diving | Roatan, Bay Islands, Honduras | PADI
It's $25 (fixed) for a cab from the airport to the West End. Local rides are negotiable. I paid $15 once to Coxen Hole (about the same distance) and $6 to Anthony's Key to dolphin dive. We didn't spend $100 all week on cab rides and there were 7 of us. You can also rent scooters in the West End - or bikes. No one really drives though due to the liability issues. The West End is only about a mile+ long so you can walk everywhere - we did. All the nightlife and most of the restaurants are in that area also.
Some other good operators - West End Divers, Reef Gliders, Sueno Del Mar - probably a dozen more within walking distance. Not a lot of shore diving though - 6 of 9 of these are off private property. #'s 8-9 would be walking from the West End:
Scuba Shore Diving Site Listing for: Honduras, Roatan, Caribbean
There's no shore diving on Utila accessible from town afaik - most of the shops/hostels/hotels are off the port in town. The three AI's all have shore dives but are private and across the lagoon channel. On your budget - out of reach. IDK what dives cost on Utila since we stayed at an AI.
Considered Cozumel? It's high on my list of cheap places to fly to for a week of diving. Do the bag drag from Cancun to save even more. Some friends stayed at Villablanca Garden outside of town (2mi.) for $700/wk. - lodging and 10 boat dives. Maybe with breakfast. There's an average shore dive in rubble just off the property. Plus 3 dive ops within walking distance.