so he works there part time, the employer approaches him
and says they're interested in taking him on full time, they
list new responsibilities and ask him for salary expectations,
you're suggesting if the number is way too high they tell
him to clean out his desk?, I'm sorry, but that's ridiculous
You may have gotten the wrong message here, he won't get fired, well I hope not as he's part time. But there will be point where he will price himself out of a possible full time position, and he could jeopordize losing this position by having a different person fill this position that he was asked to fullfill.
And yes, just about everyone is replaceable unless you work for yourself. I'm not sure what you do for a living or if you work for a company, but I'm sure most companies will continue to operate with or without you. Yes, they may experience some bumps along the way, but it's not like any company will stop dead in their tracks just because of you.
You also have to consider the current economic times we live in where unemployment rate is high, over qualified people or professionals are competing for jobs at very low wages or salary.
I'm not even sure where Desoto Missouri is, but I'm sure it's not San Francisco or NYC, where $55,000 isn't really considered great. But might be a great salary someplace else.
I think there has been some great suggestions as well as some bad ones. I just hope that the OP just uses good judgement. Obviously, during research such as what is the average income for the area you live in versus the national average, what this particualr proffession normally pays are figures that will be very helpful to come up with a fair number.
Like I said, for entertainment purposes only, I have set the over under at $42,500. I believe based on his geographical location, his age, etc, I don't think his salary would exceed this figure.