There is loads of ways to liven porridge up if you can afford to make it a bit more interesting. I like it with grated apple and peanut butter, or carrot cake style with grated carrot, mixed spice and sultanas, or with banana and Nutella. So many options.
Toast with peanut butter is really filling. I like to add extra toppings, again depending on budget. Banana works well, with a drizzle of honey and a sprinkle of seeds. Apple is also nice if you slice it finely. Works will work a pinch of cinnamon as well.
Cooked breakfast items can be cheap if you just do a couple of things, eg. Bacon and grilled tomato one day, fried egg on toast the next, hash browns and baked beans the next, sausage butty the next, etc.
Scrambled eggs, I make these with leftover veg sometimes some onion, pepper, courgette, mushroom, whatever there is. Sometimes I'll add grated cheese, or some ham, bacon, etc. If you don't like scrambled egg, mini egg muffins work well as well. I add a little bit of milk to make the mixture go further (as eggs can get pricy if your using a lot).
Other alternatives would be baked oats, overnight oats, savoury muffins, vegetable fritters, pancakes. Flour gets a bad rep, but is a great way to save money and make things more filling by baking. If you don't want to use traditional flour, corn bread is lovely and is delicious for breakfast warm with butter, or you can add other things I like crispy bacon and maple syrup American style. Scones and rock cakes are also simple to make, both can be made fairly low sugar, probably less sugar than most breakfast cereals. Or you could experiment with some pastry, my favourites are cheese twists and cheese, tomato and bacon turnovers
I love smoked salmon trimmings for adding a luxury element which isn't expensive. Great in scrambled eggs, baked in muffins or as a breakfast frittata, or used on top of a bagel with cream cheese. Really nice on ryvita as well. Kippers are a great option too if you can cope with the smell. They don't take long in the microwave and are great with a piece of toast to soak up the smoky butter.