Should the invention be made open source if patent is not affordable or patent the invention? Which is the best approach ?

Yes, if you can't afford costs for preparing, filing and - eventually - prosecuting a patent application for your invention, or don't want to protect your invention for any other reason, you should consider "to make it available to the public by means of a (written) description" such that it forms part of the state of the art.
If you don't "publish" the invention, a third party making the invention (independently) and obtaining a patent at a later time could stop you from practicing the / your invention!
There are different ways to publish the invention. Some prefer publications that are generally difficult to find, but can be relied on in case a third party tries to stop them from practicing the invention.