I have an obsession for collars too I'm always buying them, for me though I love to find something different.
So if you could incorporate some unique charms into the design I think that would be a good idea, also more personal.
I think the matching leashes would be great too.
For me the price would depend on what materials were used and the overall quality. You could do a combo price, say like "if you add on the matching leash there's an overall discount."
I think the price also depends on where you are selling them, like at a craft fair or online. As long as it's conpetitive with similar items and suits potential customers it should be successful