When training combat, one thing everyone needs is food. I'm gonna talk bout the higher level foods (Tuna and up) and some rather unorthodox types of food. First: Tuna. Heals: 10 Hp. GE value: 78-82 gp. Obtainable at lower levels ? Yes. Tuna is common for lower levels who can't afford (or fish and cook lobsters) some higher level players use it when fighting weaker monsters to save money.
Second: Lobster. Heals: 12 Hp. Current GE value: 175-184. Obtainable at lower levels ? Yes. Used by those with low and high levels, lobsters heal a moderate amount and are freely available. Used by everyone as an affordable food.
Third: Monkfish. Heals: 16 Hp. GE value: 274-304 gp. Available at lower levels ? No. It heals a significant amount and needs a quest, higher levels players who can afford it use it to train or fight larger enemies.
Fourth: Sharks. Heals: 20 Hp. GE value: 727-805 gp. Available at lower levels ? No. Commonly used by high level players to fight bosses and very large enemies. Save these things up for Boss Fights and not training.
Fifth: Basket of Strawberries (I'm not kidding). Heals: 25 Hp per full basket. GE value: 285-300 gp. Available to lower players ? Probably. These heal a lot, save space and are best used for stuff like agility where you only need to heal small amounts of damage. The only problems are the fact that they take time to get unless bought and they heal only 5 Hp per berry. They're also cheap.
Sixth: Chocolate Bombs. Heals:15. GE value: 109-266 gp. Available to lower level players ? Probably. Heal a ton, can be cheaper than lobsters and unote at general stores for I think about 2gp. Perfect for Dagannoth training but are limited in availability.
As you can see, more exotic foods (Strawberries and Choco Bombs) are less expensive but are more difficult to obtain. Common foods are easier to obtain but more expensive. I hope what I have written here has helped you somewhat.