True, I think I'll change the 30,50,20 (would be sunburned, would have gotten it elsewhere, would have stayed in shadow) to 20,60,20 (would be sunburned, used own or gotten elsewhere, stayed in shadow).  

Thanks, that is really useful information! 

I think I'll wait for a bit more feedback to come in and then update this post with the new information. 

Thanks for the feedback! About the comments:

1. I skipped over a factor there: I expect 1/2-2/3 of people to already have sunscreen in their group and likely using their own instead of a random one given by a stranger (probably also somewhat lower quality as we bought a generic brand for in this calculation). But whoever does not use the sunscreen is also not part of the cost calculation. The 30% is the sunburn risk of those who would use the shared sunscreen. If my 1/2-2/3 having sunscreen in their group assumption is correct and 30% of those that do not get a sunburn, you would expect 10-15% of people to have a sunburn at the end of such an event, of those being there early with you in the queue. Lots of people also come later in the evening, which decreases the fraction of people with sunburn you'ld see further. 
2. I might be a bit off here, but the 0.035 for lower back pain seems a bit low. I think most people offered to have another day in the month but lower back pain the whole month would decline. On the other hand I'm pretty sure that most people with lower back pain would take the trade the other way. I also found this for GBD 2017, which would be a mean DW across the varieties of low back pain of around 0.11. The 0.035 would seem like a reasonable weight for the median "back pain" but not for the mean including more severe cases which is closer to my association with the term "low back pain". Table in the link: