So.... a staffing question this time....

I'm running the numbers to project if I hire a part time marketing coordinator, what my sales volume needs to be to support that expense. I've checked the local salaries for that job, extrapolated it to a part time wage amount and came up with $19.25/hour or roughly $2000/month (roughly estimating FDIC and taxes). Obviously I need to bring in that same amount to break even on it. What I'm not sure about is if this person will bring me the increased sales that I need. My sales are currently about $150/month (yeah....). Is it realistic to expect an ongoing, polished, professional marketing campaign to bring my sales volume up that much? Or rather, i should say HOW LONG will it take to see that level of increase in my volume? I don't want to put numbers in next year's budget that are complete bullshit. :)

(Not sure if this belongs in the marketing forum or here.)