Well, you can either install Limesurvey on your own server and pay for the server, or you can purchase responses from Limeservice. They host for you and keep the installation secure / up to date, so you don't have any hassle.
Have a look here, it is explained pretty well:
www.limeservice.com/en/pricing
Free version:
Run professional surveys for free!
You can run an unlimited number of surveys and collect up to 25 reponses per month for free - with no limits on the number of questions! At the beginning of each month period (starting with your register date) we will stock up your response account balance to 25 reponses again. Please refer to our Terms and Conditions for further details.
n=500 responses are $35 according to the link above.
It is up to you. Either you install/update/host on your own server (and pay for the hosting), or you go with Limeservice (they host/update/secure/troubleshoot the installation for you).