The preposition "за" has a different meaning. One of its meanings is "in exchange of something". You give "спасибо" in exchange of our answers. So, the right way to say it is "спасибо за ваши ответы!"
If you pay for something, you give money in exchange of what you buy: "заплатить за обучение" - to pay for education.
"Он платит мне 10 долларов в день за мою работу" - He pays me 10 dollars a day for my work.
"Я хочу вас поблагодарить за вашу помощь" - I want to thank you for your help.
You can also use "за" in noun+noun combinations:
"деньги за работу" is "money for work" (money which is paid in exchange of work)
"5 рублей за билет" is "5 roubles for a ticket"
"благодарность за услугу" is "gratitude for service"