Ok, technically I didn't pay anything to kayak, except maybe $5 in gas money to get me there if we want to get technical. I own my kayak and paddle & life jacket, so I had it to bring along. We went kayaking at Sakatah State Park in Waterville, MN. State Parks in MN do have an admission free of $5 a daily or a yearly pass for $25. So, if you had your own kayak & supplies you would only pay $5 to enter. We have a yearly state park pass, which we have used many times over now so that our cost per use is probably around $3 right now.
My friend who I went kayaking with did not own one, but she could rent one from the park office (pictured above) and the cost of that came out to less than $5 for 2 hours. It was $10 for up to 4 hours (so, you can't really pay for 2 hours) or $20 for 4 - 8 hours. It is the same price for a canoe that you can have up to 4 people in, so in that case you would end up paying $2.50 a person for 4 hours. Good deal! All paddles & life jackets are included.
Of course there are other places you can rent kayaks and canoes from, but I am giving you an idea on a place to do it that helps you save some green in your pocketbook.