Hello ! I know I'm a bit late for any advice, but just in case you (or someone else reading) finds this useful...
When I decided to have an iui I didn't know the low success rates (10-20% depending on maternal age). I'm glad I didn't, or the process would have been so much more stressful !
I have no fertility issues, had an iui because I'm single. Turned out to have a 50% success rates (1 ds, 1 early mc, and just got a bfp after the last attempt twelve days ago, so fingers crossed on this one). So, three bfp out of six attempts. No hormones at all, not even a trigger shot, I just used ovulation tests.
So, iui can and does work, however I believe success rates when there are fertility issues would be much lower. Like you said, it's like ttc minus the sex. The one difference being that they wash the sperm beforehand, so that can make a difference.
Financially speaking I pay 2500€/attempt, so my son cost 5000 and number 2 (if he sticks around this time) will have cost 10.000 (including flight tickets, hotels, donor sperm and the iui procedure itself).
Like other people have said the costs of multiple rounds add up quickly, I think number 2 will have cost as much as one ivf cycle. The big difference for me is that this way I didn't have to pump myself full of hormones, which is always a good point. Plus, ivf/iui is illegal in the country where I live currently (somewhere around the middle east) so it'd have been hard to get a medical follow up prior to insemination. Or else I'd have had to book 3 weeks vacation in the country where I'm having the procedure done (which I could have done maybe once/twice, but not multiple times)
For people with fertility issues, I think moving on straight to ivf is a good call, unless the infertility is solely male factor. Also, the max efficiency of iui is around 3-4 cycles, so if you don't get pregnant after those I would move on to ivf too.
Hope that was helpful, wish you the best of luck !