My friends and I usually split the bill equally so everyone pays the same amount no matter what we bought.
But if someone bought something for $7 and another person bought something for $25, then we'd each just cover our own cost.
And if someone didn't have enough money to buy something, we'd just all pitch in money and buy them something.
You should never just make one of your friends pay, unless they say it's their treat. Then you are, of course, expected to treat them the next time you go out to eat.