I would agree with YU on the concrete wall, but Im not sure the cost involved in building such a thing.
If I were to bulid a wooden wall this is how I would go about it.
Dimension wise I would start with a 12 foot wide wall and would use 4x4's for my vertical posts and space them every 12 inches. I would need 13 posts for a 12 foot wide wall. At a little under 15 bucks a piece here in MD for pressure treated 12 foot long 4x4's just estimate 200 bucks for the posts. I dig holes and bury the the posts at least 3 foot deep. at 3ft deep it should be sturdy enough with the sakcrete (3 bucks a bag) to give you a 9 foot high wall.
i would then mount 12 foot long 2x12's (need 8 for a 8 ft wall) horizontally. at 20 bucks say 180 for 9 2x12's. if the posts are 9 foot you can leave the bottom fott open or use another 2x12 to close it in. I would just use a board ont the bottom so the balls wouldnt go through. although I wouldnt worry about plywood covering the bottom foot of the board.
then i would mount 3/4 inch plywood (sheets are 4ftx8ft) sing alot of wood glue and 3 inch long screws (minumum) to penetrate into the 4x4 posts. 3/4 inch plywood is about 50 a sheet so 150 bucks for plywood. before i did anything I would probably cover the posts and 2x12's with truck bedliner (that stuff is just about indestucable) or at least get some weather proofing treatment and a few good coats of exterior paint.
This wall would be practically invincable and could easily survive a hurricane
to sum up costs of materials for a 12 foot wide 9 ft high wall using the upper 8 ft of wall as hitting surface and bottom foot open (or closed) these are estimated prices based on lumber costs in 21028 zip code
qty 13: 4x4x12's = $170
qty 9 : 2x12x12's = $180
qty 3 : 3/4 ply = $150
qty 13 : sakcrete = $39 (1 bag per post is probably overkill but better to haveto much than too little)
qty 1 : gallon wood weather proofing $30
qty 1 : gallon exterior primer and sealer $20
qty 2 : gallon exterior paint flat base $46
qty 4 : liquid nails heavty duty construction adhesive $20
qty 1 : caulk gun for liquid nails $5
qty 2 : 1lb box 4 inch exterior wood screw $16
total for all that is about 680 bucks, but trust me that wall would not move for a lifetime and as long as you kept it clean and applied a new coat of paint every couple years it would last.
or you could say screw the effort and drop a couple grand on one of these
Fiberglass tennis backboard