Many, many years ago, my boss (who is like a second father to me) told me this when we were discussing salaries and cost of living:
As an employee, your job is to try to make as much money as you possibly can. As an employer, my job is to try to pay as little as I can. Then we have to work together so that no one feels shorted. You want a wage that you feel reflects your responsibilities and contributions to the company, something that makes you feel valued. I have to figure out what is the smallest wage that I can pay you that will allow you to have a decent level of living, and also keep you happy so you don't go looking for a better income. Sometimes that middle number isn't possible to find where everyone is happy. But in the end, you have to look out for what you feel is in your best interests, and I have to look out for what I feel is best for the health of this company. I'm sure it would be great if I could pay everyone $50 an hour, but the company would go bankrupt in a month.
While it sounds a bit cold, it is very succinct and true, it's just vocalizing it in a way that no one really wants to think about. If you work for a big corporate company, I don't think you can even ask for raises usually, I think it comes with your regular reviews. If you work for a smaller more personalized shop, it tends to be a lot less formal, and irregular.
All of that said, I am in the same boat as you. My official position is Prepress Manager. My responsibilities include: Prepress manager, IT manager (including all pcs, macs, smartphones, copiers, CTP, and anything else I need to fix), Color Management, Web design and development, MIS system manager, new equipment and software Research and implementation (well, anything that uses a computer anyways), Packaging and die designer, and general boss's right-hand back-up. I also generally repair/fix minor issues with all prepress equipment, copiers, CTP, and sometimes other things. There is NO set "position" to cover what I do.
in the midwest, general "good" pay for this kind of position is $20 give or take a bit. But just a prepress tech generally maxes out around $18. Sometimes if you are doing a lot of CSR work, you can maybe negotiate a small percentage commission.
If you really like the company you work for, be prepared to work with your boss, maybe go in without giving him/her a number at first. Demanding a high salary right off the bat is a good way to put them on the defensive and feel backed into a corner. I think the suggestion to put together an example of all the things that you are responsible for, with standard wages, and then discussing from there would be the best bet. Just don't forget that no matter how important you think you are to the company, everyone can be replaced.
P.S. I did get a raise from that conversation with my boss. And I don't think I have ever asked for a raise since. They usually just appear on my pay check. Generally after I have done something beneficial for the company, or learned a new valuable skill. I was very fortunate to start working for one of the only owners I have ever known that actually values his employees at 18.