Expect about $50-$60 per hour from any serious professional, it could be higher. Ask to see a portfolio.
Based on other posts you have made Shid, it looks to me like you want to start a sports apparel business. I suspect that you are wanting to purchase the rights to the designs as well? You did talk about a hand drawn sketch, ( I am assuming it is done by you) turned into vector ready artwork? You can go one of a few different routes.
Purchasing the rights to any artwork is always going to be a bit more expensive than not. Make sure you get a "transfer or ownership" contract with any designs you want to own. Like it or not, any artwork a designer makes is their property, even if it's your idea.
1) Hire a college kid. Put up a flyer at a local college saying you will pay for your sketches to be vectorized. Put up a price you think is fair. If you get no responses, up the price. A kid in school will work for less, and can use the design for their portfolio.
2) Post a "Graphic designer wanted" thread here (specifically here in the referral/recommendation section). There are a bunch of talented designers that check this section daily and will contact you. Ask them questions, tell them exactly what you want, find out what they charge. Go with the one you think is best for your needs, not the cheapest, the one you think that is the best fit.
3) use a service online like these guys
4) use an online competition site. You post a job, describe it and a bunch of artists do the work, you pick the winner. site 1 site 2 site 3
There are many many more of these kinds of sites out there.
5) purchase clip art
and get really good at putting designs together.
These are a few options for you that may work. Keep in mind, like everyone else said, there really is no "standard price" for design work. The simple reality is, a design that takes 3 hours to do will cost less than a design that takes 18 hours to do. I do not recommend going with "cheap solutions" if you are planning on starting your own sports line brand. It's very important to remember, tshirts are a heavy design based product.
Hope this all helps.