The fish is cheap; the water is the subscription
Angelfish themselves are inexpensive to buy and are modest eaters, so first-time keepers are often surprised that the species sits mid-range for running costs. The reason is the tank an angelfish demands: a tall aquarium of at least 29 gallons, and preferably more for a pair or a small group, kept warm in the high 70s to low 80s Fahrenheit, filtered around the clock and lit for most of the day. Nearly every recurring cost flows from keeping that volume of water warm, clean and moving in a room that is cooler than the tank. When you budget for an angelfish, you are mostly budgeting for its water.
Electricity: the steadiest line in the budget
The heater is the biggest single consumer, cycling on and off all day to hold tropical temperatures, and it works hardest in winter and in air-conditioned rooms. The filter runs continuously, the lighting runs on a daily timer, and in a large setup or a cool room those three together outspend everything you will ever pour into the tank as food or chemicals. Tank placement is a genuine cost decision: an interior wall away from drafts and cold windows noticeably reduces how hard the heater works compared with a spot against an outside wall in a chilly room.
Food: quality over quantity
Angelfish are cichlids with a protein-leaning diet, and they do best on a good staple flake or small pellet varied with frozen foods such as bloodworms and brine shrimp. Frozen food costs more per month than dry food alone, and it is worth it — angelfish kept on a single basic flake tend to look washed out and grow slowly. Because they are tall, deliberate feeders rather than frantic ones, very little food should reach the substrate; feeding only what disappears in a couple of minutes keeps both the fish and your conditioner usage in better shape.
Water, media and the consumables drawer
Weekly partial water changes on a tank this size use water conditioner steadily, and the larger the tank, the faster the bottle empties. Filter media needs periodic replacement where it is the disposable kind, test-kit reagents run out, and algae scrapers, nets and the occasional replacement heater or bulb arrive as small, irregular costs that a monthly budget should smooth over rather than be ambushed by. None of these items is large on its own; together they form a predictable second tier of spending after electricity.
Keeping the month under control
The sensible levers are boring ones: a well-fitting lid to hold heat in, a timer so the lights never burn longer than the plants need, food bought in sizes you will finish while it is still fresh, and maintenance done on schedule so small problems never become treatment-and-replacement problems. What you should not cut is the heater's quality or the water changes — those costs are not overhead, they are the fish's welfare. Setting out the whole cost picture first, while a smaller setup is still yours to choose, is well worth doing; put a follow-up question to KinsLe's Kinny; he answers using the KinsLe Answers library.