I think of panini in three grades:
1) Those made far in advance and waiting for a tourist or non-discriminating person to purchase. They have little content, taste and the bread is made edible only by toasting it. Not worthy of a photo here.
2) Those made daily and containing ample portions, tasty fresh bread and not located right next to a tourist hot spot like the Ponte Vecchio in Florence. They may be take-out only, or have a bar where you place your purchase along with a drink.
3) Those freshly-made from a menu or by creating your own. They are pre-priced or sold by weight and usually run from 2-4.5 euro. You can find them anywhere from a store selling meat and cheese products to a hole-in-the-wall stand with a line waiting.
And also under the third category I Due Fratellini – a very small stand on Via dei Cimatori near Orsanmichele Church in Florence. I regretted to pass it by on this trip but it had always had a huge line. They have almost 30 kinds which are ready in a couple of minutes. They start at 2.5 euro and are quite good (along with their wine). Nowhere to sit, but they have a numbered rack for you to rest your beverage. Run by hard working brothers who probably drive new Lamborghinis.