You have two choices. The cheapest choices is to learn to use a font tool and do it yourself.
The best source I know of is a company called FontLab who produce a variety of applications at all level of price and capabilities:
Or, you could hire a type designer, or a designer familiar with type design.
Hiring someone like me or any other type designer is the most out of pocket expense. Doing it yourself or another person in the company is probably the least expensive option even with their salary or wages factored in. There is a learning curve.
Aside from Fontlab, another great editor (though on PC) is Font Creator Pro. It's the one I use the most and is more cost effective.