In my opinion youtube is good for making people aware of a new product being released or increasing brand awareness.
Being a small company with limited production budget I haven't used youtube to market but I have done similar viral marketing which works on similar principles.
We had the contract to build a website and market a world cup song (i'm not saying which one) on a really tight budget. We ceated a funny flash video and a flash game on the site and made a box on the page that people could type their friends emails into to link them to the game or animation. The email got circulated around the usual office crowd and within a week we were receiving 4,000 hits a day with £0 spent on advertising.
Another thing this reminds me of is an american based design company called jib jab who were among the first companies to hone in on viral marketing through flash animation. They released a series of funny animations and received publicity worldwide.
In short i find that when doing these things with a low budget it works best to do something topical in your related sector that will get an instant reaction.
If anyone out there is interested in finding more about costings of projects like this then I suggest talking to your web design company. If they don't specialise in this area then you could always send me a PM (Shameless plug I know.. hope its allowed

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