It sounds like a really good idea for the boarders, but it seems to me that keeping track of all these things would be a huge headache for YOU. Have you considered making different packages? Example:
Full care - feeding, turnout, blankets/boots, holding for vet & farrier
Partial care - feeding, turnout, extra charge for holding ($10 per time) & blanket/boots
Vacation - full care but at a per day rate - maybe figure out what full care costs per day and then raise it 20-50%?
That way you won't have to track 6 feedings, 30 exercises, 4 holds, 16 turnouts, etc. etc. and most people will pay a flat fee rather than add things on regularly. The only thing that is good to keep separate in a non-training board situation is the exercise because I think most people will not want or need it or may only need it for special circumstances. Maybe I am wrong. I would also not round pen a horse for more than 20-30 minutes. An hour seems extraordinarily long and hard both physically and mentally. If I were a boarder I would really prefer someone I trusted to ride my horse vs lunging or round penning but I'm sure others feel differently.
Turnout is either half day (usually 4-6 hours) or full day (8 or more hours). You don't have to observe and the horse doesn't have to do anything in particular, the point is just to be outside. If the weather is bad and they cannot be outdoors, arena turnout is usually more about a horse rolling, running, bucking, and then back in after about 30 minutes or so, more if they have a friend to play with.
I am guessing that boarding costs where I live are very different than where you are and I have always had my horse in training board so I cannot be helpful in terms of cost. Your suggestions seem very low to me and I worry you are shortchanging yourself.
Have you thought about making a survey for the boarders to see what they are interested in? I could be all wrong about this and they might completely reject packages and only want a la carte. You could ask what they would be willing to pay as well and take that into account when setting prices. Make sure you have very good records if a la carte ends up being what you do.
Also, do you know how to body clip? That can be a great way to earn some money. Every November and February I regret never learning how to clip as I hand over $140 to the groom.
$35 a month for am feeding only
$10 for the second horse
$5 for the third horse
$20 an hour for exercising : I personally don't think the round pen for an hour is all that difficult for a horse. I take my time with a warm up and cool down, and we have a very large round pen so the horses aren't turning extremely tight turns. So if they are walking for the first and last 10 minutes, it's not bad. I see your point though, and I'm sure there are those on here that will disagree with me, but it depends on the horse. With a young, energetic horse, it might take them 15 minutes for their brain to return to body and actually start listening. You're right though, for some horses that would be a lot, but I anticipate most of the horses only doing half an hour.
$5 per turn out : half an hour. We don't have an area for half or full day turn out. All of the horses have a smaller out door area, so the point here is more to let them stretch their legs, move around, and exercise themselves. I can't tie up the arena for 4 hours with a horse turned out.
So that's where I'm at right now.