You can go cheaper, just don't go so cheap you're underselling and burning yourself out. No lower than $10 for a sketch, even just a chibi one. Go higher for ink and color if you want to offer that.
Realistically, yes, it's going to be more difficult for you to make a profit with original only. But, even when people do offer fanart, their first couple tables often are either at a loss or they break even. It's normal, especially because you have a lot of upfront costs in starting inventory and display. I was in the red for my first con despite doing very well, but that's largely because I got way too much inventory... don't be like me xD
Original stuff does best when it's either content (like a comic) or broad, general appeal items. Like Joanne mentioned earlier, pets are great. A lot of people do themed artwork like zodiacs or dragons, or they'll combine things to make something new (one person I know makes lolita cat illustrations, for example).
There ARE people who can sell original art regardless of concept, but they have a very strong aesthetic and design sense to rely on. I don't think you're quite there yet (heck, I'm not) and that's a pretty specialized niche.
When people buy original stuff, it's usually because they relate to it somehow or consider it a label to their identity. When designing something, think about who the customer IS and put yourself in their shoes. What would make you buy something? What labels do you associate with yourself, and what imagery is common for those labels? Once you can hit on that, you might have something really worth selling.
