How big of a raise should I ask for?

My job is complicated, and it's company policy that nobody knows anyone else's salaries. So I can't just ask someone else what they get paid and ask for that.

I work for a large indie bookstore. We're the largest in the state, and usually have upwards of 50 events a month (signings, local celebrations, workshops, etc), so we have some profit. The company is also doing well, so we're not hit bad by the bad economy like some places.

Here's the thing: I was hired 2 and a half years ago for catalog entry. I just did nothing but data entry, part time, while I was in college.

Now, I have my bachelor's degree in an unrelated field (Philosophy), but it is a Liberal Arts degree, and it is from a good university.

I've also taken on way more work, and much harder work. I write event materials, do research in order to write the materials, I do graphic and web design (yes, I know CSS and HTML), design email campaigns, and put together monthly event calendars. Those events are one of the biggest sources of pure profit for the store.

My official job title is "Administrative Assistant", but since it's almost all marketing now, I'm more of a "Marketing Assistant". Nobody else in the store does all the work I do, although many people could if I left. There's nobody else with my job title.

I started at $7.50, and I'm now paid $9.50. I was paid at the rate floor staff (we are a brick and mortar store) before. Now almost all of my work is back office, and I have significant responsibilities. I have to meet press deadlines, design things for a large email list, and I have to sit in on meetings (and I do have some decision-making authority in those meetings as well as input) every week or so with people who've been with the company 10+ years, our head web designer, the owners, and the general managers.

The bottom line is that I was hired as basically a grunt, and now I'm really only friendly with managers and office staff because the floor all assumes I'm a manager myself. I have to make decisions, and my work doesn't stay in the office. Yes, I sign in at home all the time.

I'm still working part-time, but our head PR guy is working too much, and I have the experience to take over a lot of his workload. I can only do this if I work at least 30 hours a week. I want to go full-time, but I can't at the rate I'm paid now. I keep my enrollment up in graduate courses to full-time (which is why I only work part-time) so I get financial aid. If I drop my enrollment to part-time, I have to be making A LOT more in order to make ends meet without my aid.

The cost of living where I live is high-average and fairly urban. The state I live in pays poorly, but the city I live in is one of the more expensive ones in the state. Our property values declined by about 40?rom their highs though. I don't live in the South or Midwest, but I don't live in a coastal city either where the cost of living is really high.

So, how much should I ask for?