Why surveys are so expensive?Not sure. Maybe they have a monopoly on the hosted aspect of it and feel their fancy graphs and analytics make it worth more. The more challenging aspect of surveys is handling email and not being blocked. I would just dole that out to an email marketing company assuming the email addresses are not tied up in a contract that forbids sharing them with a 3rd party and assuming that 3rd party is trustworthy. Many are not.
Do most companies write their own survey software
I've seen it done. Less bells and whistles but it gets the job done. Some employees don't like answering internally hosted surveys for the risk of not really being anonymous, if that was the goal. I suppose it depends on who your target audience is.
is there anything cheaper or are you using some open source?
There are some old perl CGI survey scripts out there but I do not have a link handy. If you don't need all the bells and whistles fancy graphs, analytics then those old scripts may work just fine if you just want structured feedback on something. I'm sure some of the developers here could write a survey form in {favorite language of choice} in their sleep.
Honestly I think just sending a well formed email to everyone and get free-form answers may work for some use cases. Even a dedicated chat channel can work if people do not need to be anonymous. If the survey is for customers I think email or web forms could work but probably takes more time to consume but then you may get more honest answers and more details that the survey may have missed. Customers appreciate human interaction especially if someone responds to them and lets them know if/when their feedback resulted in changes.