Most people think of Open Source software as 'Free'.   In any case, it's very budget friendly.

A lot of the software began as graduate student projects or similar type projects.   A small group or individual needed a software for some reason and put in an application for a government grant to pay for the development.  If the grant was paid for by the government, then the government (or by proxy, all it's taxpayers) then it ends up owned by the government (or, again, all it's taxpayers).    So while the software may be sold, it must be offered to it's constituency for free also.

Any of the software used by Schools Open is still under development.   That means that there are people who are still updating, maintaining, fixing, patching, and making it better.  All of these people have a vested interest in the software being successful because they are using it and have their name attached to it.   Development can happen at a by-the-minute level.   In some ways,  it can be more up to date that very expensive software packages from bigger retailers.

Schools Open is here to help school districts in any way possible.  If that means custom code, then it will get done.   Software must be updated from time to time,  which means the custom code also.  Generally Schools Open will avoid custom code,  as it is the goal for small schools to be able to function on their own and not be tied to Schools Open in an ongoing basis to update this code.

No.  It's curious where this idea came from.  Remember that it was developed by folks who needed a problem solved.   At least some of those were windows people.   So they wrote it in such a way they could use it on their machine, in a way they wanted to.  If it could be measured,  it would not be surprising to see that there is as much Windows and Mac software available as linux or any other operating system.

Software Schools Open is free.  Labor to complete a project is billed on an hourly basis.   You pay for the time it takes to set up your project.   Nothing more, nothing less.

Linux is one of two base operating systems, originate in part or full by University of California Berkeley in the early 70's on a government grant.  Ergo,  it is free to the general public.   It is frequently the base operating system used by product developers,  and it is a VERY capable operating system for most users.   Lots of the projects you might like will run off of a linux computer.   Frequently those can be managed by web interfaces and little or no linux experience is necessary by your staff for day to day operations and maintenance.  All that in mind, switching all of your computers to Linux may not be so effective for your district.   Even still, a school district based entirely on linux devices is in the future.   Someone will blaze the trail.