Back to OP... Yup, you broke the law and now must pay. As far as how much, I think every municipality gets to set their own price. I'd imagine it's anywhere from $20 to $80 per dog. I also have a dog that I let "off a physical leash" though he is always electronically leashed. However, some municipalities don't let you use the electronic leashes. I always take into consideration what my action may bring if I let my dog off a physical leash.... however, he is ALWAYS ecletronically leashed...
See what I'm doing there? Ha. Pay up and leash 'em up. You never know what may peak their curiosity off leash. As calm/collected/nice/trained/insert adjective here your dog is, other people don't know that. I LOVE dogs, but I know that if I see a dog charging me or mine outside of a dog park or human control, I'm likely to put it down.