DITTO RBG and Imafan
Before even discussing planting cost I must ask if you have considered mature size of your plants. Do you really need 12-15 plants? If the diameter of a mature plant is 5' then the center of each plant should be 5' from the center of the adjacent plant. Mature diameter 8' then 8' foot centers. If you are planting too close looking for instant privacy you will end up digging plants up in 3 to 5 years. That is a VERY costly mistake that DIY landscapers frequently make.
RBG is correct. The hole should only be as deep as the existing root ball but several times wider. The larger the root ball, the wider the hole. A container grown plant with a root ball 15" in diameter needs a hole no less than 30" wide.
When I had my landscaping business I frequently planted much larger tree. I rented an excavator. Trees with large root balls required holes 3 to 4 times wider.
LSU Ag Center has done lots of research on tree planting. One of the considerations was amending soil or not amending soil. They actually did comparisons and found that trees planted in nice, soft amended soil ended up with the roots staying in the nice, comfy hole instead of spreading out. Trees planted in nothing but native soil had much more substantial root systems which of course meant a stronger, healthier tree.
Finally - do not fertilize until the spring of the year after planting. Fertilizing pushes foliage growth which puts stress on the root system. You want a well established root system.
RBG's suggestion of a "planting party" is a good one as long as everyone understands how to properly plant the trees. Don't waiver and don't fall for "well this is how my Grandfather did it." If you have friends and family with sharp shovels and a willingness to follow directions then have a party!
Imafan's suggestion of getting an itemized cost break down is a good one. You do not need or want amendments or fertilizer. Also discuss planting techniques - hole depth, width and spacing.
Whether you plant the trees yourself or pay someone to do it you are making an investment. Make sure it is done properly. Unfortunately there are so called "professionals" who take advantage of landscaping "newbies".
Cost - When you say $500 per tree is that just the planting or is that providing the trees as well? The cost of trees is less about height than about root ball size and trunk size.
I am really concerned that you are planning on planting too close.