I've done 50 or so giveaways for mostly accounts for a game and some cash and gift card types. While almost all the accounts were free donations from viewers that wanted a shout out, I did giveaway one that I spent about $400 on and used in a 13 part series for the game building it from the start to the end.
The giveaways were not to attract subs as much as they to help branding my image on YouTube as one who contributes to the community. I'm also just terrible with money since I don't care about it more than the means to an ends it is, so if I have it's gone, and giveaways do a few things.
Now you're going to have to have a decent channel size to start with. No reason to giveaway stuff when no one is watching; however, if you find another YouTuber to work with, and get him or her to post an annotation for your video, you will see results. There are some other factors of course. Such as the contest needs to be relative for what you post and what the person promoting you is posting. If it is, and you're channel art is good, avatar is good, and basically you can portray the image of being a professional, then yes it can help a ton.
However, finding that person to promote is the hard part. I have used that method to promote a couple dozen youtubers and it saves me the hassle of having to actually deal with a giveaway now.
Also be sure to use a sight like rafflecopter since the comments will simply be filled with bots if you don't. It's a win/win since you can get your FB, twitter and other social media exposure with the giveaway by making the options to enter, follow you or like your FB page.
Don't be in a rush to get subs. Just enjoy what you do. If you have a passion for YouTube and making videos, you have a shot. If you're looking for fame or money, you're better off playing the lottery. Best of luck!