I say it is VERY event dependent but also demographics. I've shot equestrian (my daughter rides dressage so I shoot at her events) lacrosse, football, soccer, wrestling, martial arts, dances, parties as well as ballet and choir. Some at high school level some younger and some post high school level. So I have shot many different events in many areas of many age groups. But my pricing is the same for all so that is not a factor when comparing my event sales to each other.
Your numbers of 8-10% I would consider very good for event. But like mentioned, if your price is such that you are practically giving them away then what's the point? Personally I do not even think about the percentage of images sold. I think about how much time I spent shooting and processing and then how much money I actually made on the shoot. If I can come in around $25/hour after all expenses then I consider it a good event shoot. I define my event shooting as not being paid to show up, just making money if people buy after the shoot. I also think event shooting is typically tough to make money at. There are a few events I am paid for to just show up and then I make more on print sales, but most are not this way... this is starting to change as I am becoming more known and people are wanting me to come shoot their events.
Of all the event shooting I've done over the years I've seen sales all over the map and I have yet to figure out why some sell great and some almost not at all. My equestrian sales tend to do better than most other events but martial arts has been my best so far. Going by my success or failure judgements based on hourly rate earned after sales I've earned as little as 50 cents per hour and as much as 50 dollars per hour and everything in between for no money up front event shooting.
Now I also do other types of photography that pays better, like portraits, weddings, etc. But I really love event shooting and one of my goals is to figure out which one's are worth it and why some work and some do not. I know that marketing plays a role, but there's also something else at play that I have not figured out. There are many events that I shoot where people tell me they are going to buy for sure, and never do. And other events where I feel it will not go well and they surprise me and order way more than expected.
So in the end if you figure out your hourly rate after all your time and expenses are figured in and you are happy with that rate, then you should always go after that event and keep shooting it. If the rate comes out lower than you like then don't shoot it anymore.. unless you are just shooting it because you love to shoot it. For the events I shoot that do not do well I often times get requests from people to come shoot it again. When I get these requests I inform them that this event was not worth my time based on sales from last time and if they want me to shoot it again they will have to come up with an upfront fee to get me to show up and shoot. This fee is usually based on how much time I feel it will take me to shoot and process, as well as drive to and from and I figure in expenses. Some people have agreed and come up with the fee, most I never here from again. So be it.