I wonder if it would be helpful to help them see the big picture. My previous employer (a non-profit) loved to send us annual letters with a full listing of all of our benefits with monetary amounts noted.
$XX,XXX Annual Salary
$X,XXX Annual 403b matching
$X,XXX Health Insurance premium paid on behalf of employee
$X,XXX Long-term disability insurance premium paid on behalf of employee
SS/Medicare, unemployment insurance, etc. etc.
This was their way of showing the full extent of our wages and benefits to justify why we didn't get a raise from 2010-2016. But this could be used to your advantage now. Create the same list for yourself to show them how much they have been paying you in salary + all other benefits you can attach a monetary figure to. Divide this number by 2,000 hours worked in a year to get your hourly rate inclusive of benefits.
Now set your hourly rate as a consultant. Show them the comparison of the hourly and also create an annual comparison (estimate how many hours per year they will need from you and multiply times your rate for an total annual cost to keep you on contract). Make it as clear as possible that you are actually saving them money (I assume) if they hired you as a consultant compared to them hiring a full-time replacement (to whom they will need to pay all the benefits). Gently remind them that with you, they will get someone who really knows the system/procedures and requires zero training, zero recruitment costs, etc.
Sometimes I think that they forget how this works, so I try to make it was easy as possible for them to see the great deal that I'm giving them. Good luck!!