I guess you can put whatever you want on top of a pizza, provided it doesn't need much cooking. Below are a few suggestions, based on the past few weeks.
For a start, I always put tomato on. To me, it isn't pizza without it. Use passata (sieved tomatoes), because it has the best balance of taste and texture - purée is too concentrated and thick, and tinned tomatoes are too watery and bland, unless you cook them down. Passata is cheap, and a carton at around 80p will do a good ten large pizzas (don't drown them!). Pour a little on, then spread with a spatula or the back of a spoon. Garlic- and/or herb-flavoured passata can be bought, or you can add a little dried oregano or basil. If you want fresh herbs, tear them over the pizza after you've cooked it, or the herbs will scorch and their essential oils will evaporate, defeating the object.
Next, onion. I like onion, and I think it's almost as important as the tomato (ditto for burgers, unless you use relish...). Red onion is that little bit milder, and looks nicer too. Slice thinly, and break the slices into rings, then sprinkle over the passata base.
The only other essential is cheese. Mozzarella is classic, of course, but rather bland. If using, go for pizza cheese, in blocks, rather than mozzarella di buffala campana, which comes in balls packed in tubs of water. The latter is too nice for pizza, and too moist - they'll make the base soggy (but use only this for dishes like insalata caprese, which I will no doubt discuss in a future entry). Pre-grated mozzarella is very convenient. Otherwise, you can use any mild- to medium-strength cheese that will go fairly elastic when melted, or a mixture. Emmenthal and gruyère work very well. I disapprove of cheddar, however, on two grounds: it is too strong, and it doesn't melt correctly (it tends to separate and go oily). Some people prefer very strong cheese, but in this case, the balance of flavours, and the texture of the cheese, is more important.
Beyond this, you can experiment a great deal. Olives are great - indeed, I only came to enjoy black olives a few weeks ago thanks to experimenting with toppings available cheaply from a local convenience store. Slice them. Green olives would be okay, I suppose. Salami is classic, but don't use anything too nice, or too lean. I find chorizo doesn't have the fat content to work here - it just goes dry. Also, it's best to buy the salami thinly sliced - it should go a little crisp during cooking. Bottled peppers are good, adding some sweetness and colour; drain them well, and chop fairly small (beware the liquid, which can make the pizza soggy).
Soft goat cheese, crumbled, or something special like taleggio, or even gorgonzola, would be nice for a special occasion. Pine nuts add crunch. Smoked salmon is, in my opinion, too intense and salty for a pizza, but if you used it sparingly, paired with a cream cheese such as ricotta, it could be successful. Fresh red chilli, or dried chilli flakes, can be fun if you like a bit of heat. Bottled artichokes or other antipasti are good if well-drained. A little spinach adds colour, although I find the stringy cooked texture a little annoying in this context. Finally, I love to drizzle a little garlic-infused olive oil on top. Basil oil or chilli oil would also do - use before or after cooking, or both.
Essentially, the only rules are: make sure the ingredients require only minimal cooking - no raw poultry, for example. Second, add fresh herbs at the end (the same would apply to flowers or lettuce, in the unlikely event you were using them), ditto rocket (arugula).
Preheat the oven to gas 9, or as high as it will go. I remember seeing a tv programme where Heston Blumenthal tried to create the perfect pizza, and the one point I took from it is the temperature. Traditional pizza ovens are really hot, far hotter than conventional domestic ovens, so to get anything near the right temperature, you have to push it to the max. The idea is to cook the pizza as quickly as possible, giving a nice crispness without burning. That's my attitude, anyway.
They don't take long - rather than timing them, I just keep checking until the toppings are cooked (e.g., the cheese should have melted and started going golden and slightly crispy, but nothing should be brown or black), as the bases will also be cooked by this point, if they are thin. Thicker bases take longer, so I guess maybe turn the oven down once the topping is done, and leave 5-10 minutes longer, just to be sure. If in doubt, don't put too much stuff on top, especially moist ingredients, because they will slow down the crisping-up of the bases. As with all recipes, the more you make, the better you'll be - it's more of an intuition for me now.
They can be eaten immediately, left to cool, or reheated.
And as for cost, the sky is the limit of course (you can go for foie gras and gold leaf if you feel like it), but for people on a budget like me, the biggest factors are the cheese and the meat. A bag of pre-grated mozzarella that will do 4-6 pizzas costs around £1.30-1.50, and a pack of salami will set you back £1-2. On this basis, a red onion, olive, cheese, and tomato pizza could cost (including the base) around 90p (roughly calculated), and one with salami but no olives maybe 10-20p more. One reason why I have eaten so many of these recently is that very few other home-cooked dishes are so satisfying at such a low price.