It is low, but that shouldn't be surprising. A regular old fertile couple timing things well has about a 25% chance of conceiving per month. The IUI is sort of a low tech way of trying to mimic that. It helps you get the timing exactly right and maybe gives the sperm a little boost. But it's not magic.
It's worth trying IMO (unless you have severe MFI), because it's so much cheaper and quicker than IVF. If it works, great, and if not, you move on.
+1
Yup, it worked for me on the first cycle, when I thought I was destined for much more expensive/invasive treatment. I'm grateful for that.