Jason Chan has numerous videos on The Art Department, but if you only had $16 and had to choose two tutorials of his to download I'd suggest "Thunderdome" and "Character Design vol.1". Thunderdome takes you through the process of creating an illustration from an approved thumbnail all the way up to a finished illustration that you can show to a client. My problem had always been the part between thumbnail and illustration. For whatever reason I couldn't wrap my brain around how to get from point A to B. Jason provides the answer, and it's rather simplistic; a lot of time. Obviously it's not that simple, but essentially what I was lacking was the understanding of just how long it really takes to create one of these illustrations. Being able to see the process from point A to B and more importantly being able to rewind to see it over and over again just unlocked something in my brain that text tutorials simply failed to do. There are some free youtube videos out there that may help, but a lot of them are really speeded up and so you lose a lot of the nuance that goes into particular brush strokes. Chan's general insight into his work process is also very useful. This is the artwork I produced right after watching this video, and here's the artwork prior. I'm not sure if it's obvious, but personally I feel like there was a very big leap in terms of quality between the two artworks.
Character Design volume 1 is very useful because he takes you through a variety of styles that you might use to create concept art or illustration. More importantly to me he showed how you can render lineart in monochrome first before using an overlay layer to color it in. This is not a new trick and in fact I'd heard about it before and written a tutorial about it, which frankly I'm now a little ashamed of. Again as with the previous video there seems to just be something about seeing the process and listening to Jason talk about his workflow that just made sense in my head. He also goes a little bit into how he thinks about the psychology of his characters when he creates them, and how that informs things like armor design and their poses, something which he goes further in depth into in his videos on designing Heroes and Villains.
If you're interested in concept art but have a lineart background and would like to transition from one to the other, I highly recommend all of Jason Chan's videos. But if you're on a budget, these two should definitely be on the top of your list. If you've an extra $8 and want a third video, I'd go for his "Quick Sketch" video, which deals with how to create these artworks if you're under a tight deadline.