The cost of SSL certificates used to be a factor, but there is now a free service available: Let's Encrypt
There are a number of things you need to be careful of when making a migration to https (this is not an exhaustive list):
- Every resource used on every page needs to be https or browsers will show a warning (this also includes external resources such as social buttons, ads or other widgets you might have built into your page).
- You will need to check your internal linking and transition to https links.
- It will help if you also change external links into your page to https, although this is only usually feasible for links you control directly, such as social media profiles, etc.
- Using redirects to force https should be the last step.