Hope you plant them at least 6'to 10' back away from the fence and 6' on center apart.
Now for pricing.
First of all you, need to figure out how much it is going to cost YOU to do the job. This is called overhead. Overhead includes things like gas, a percentage of wear and tear on your vehicle, ect.
Then you add in your windshield time (the time it will take you to drive to the jobsite and back. Windshield time is usually less than your normal hourly rate, so charge a percentage of that, some charge their full hourly rate.
You should always check out the jobsite ahead of time, shove your spade deep into the soil in which you will be planting... in a few places to test the soil conditions. Is it rocky, clay, loam, sandy? Will the soil need to be amended with compost?
(personal I always amend the soil, it cannot hurt the plant, if done properly, feeding the soil is always a benefit to the plant...I don't care what the so-called experts recommend, I consider myself an expert in these areas as well)
Does the area have hardpan under the topsoil? Will it drain properly? Dig a small hole about 2' down and fill with water, if it drains by the next day then it has good drainage, if water is still sitting in the hole
(your client can call you the next day and tell you if it has drained or not)
then you will have to take actions to improve the drainage situation. Is there a hosebib available and a hose close enough to water those tree's in once planted? (I always carry 2-100' hosed and my watering wand with me)
Also, check ease of access. For example. Will you have to haul those 21, 7 gal trees from your truck parked on the street into the back yard and across a field to plant them or can you drive right up next to the fence dropping off the trees close to where they will be planted?
This jobsite soil evaluation is necessary and within the scope of the OP's original question because in order to not screw yourself over, you need to take into account the planting situation before you give your client an estimate or a solid price to do the work.
Jobsite analysis + Overhead costs + Windshield time + Your mark-up on the RETAIL price of the trees 25%? 50%? 100%? I think marking up a plant 100% is unreasonable, but that is my opinion)]
+ how much money you want to make on this job.
Be reasonable here or you will price yourself right out of a job.
Consider the planting conditions when you do thi's
( if easy charge accordingly, if difficult charge accordingly).
Charging by the hour on this kind of job does not usually pay very well.
NOTE: Taking a 7 gallon pot and digging a hole in your back yard then noting how much time it takes you and charging by that time...has to be thee most ridiculous advice on figuring out what to charge for planting, that I have ever heard of. It's absurd. There is much more involved in planting a tree than that
So there you have it.
Job analysis + Overhead costs + Windshield time + retail price of the plants + % of mark-up on plants + Labor for planting including any extra effort it is going to take you in order to do it. = your estimated price to the client.
It may sound complicated and sometimes it is, but once you get experience under your belt doing it, it's like reall easy to estimate any job and you won'r lose money if you follow the rules above.