I think that the best solution could be prefabricated pieces.
You can prefabricate pieces with reinforced concrete.
For the benchs: You need a nivelated base, on the ground, and four planks for give form to the side parts. One plastic sheet, over the ground. and covering the internal side of the planks, and you will can easily unmount the formwork.
For make round corners you only need to attach a cheap plastic piece like this to the planks:
For the pillars: You need PVC tubes, that you will use as formworks for the concrete. For three or four pillars by bench, around 250 or 300 mm tubes could be a good size.
Don´t forget to make good foundations.
When you have all formworks ready, call for a concrete truck, extend and compact concrete in benchs and tubes. When concrete have enough resistence (usually at 7 days), then, mount the benchass over the pillars, using a good cement mortar.
For the reinforcing bars, you can use 12 mm diam bars, making a grid of 20 x 20 cm, and you can form handles to elevate slabs later (with a crane, for example, if you need it).
Concrete density is 2,5 Ton/m3, so you can know the weight of each piece when you design it.
When you call for a concrete truck, first you need to know the concrete volume you will use. So, measure benchs, pillars and foundations, and get the volume.
If you make 0,9 x 0,9 x 0,1 m slabs (0,08 m3 = 200 Kg), four pillars (0,3 diam x 0,8 height = 0,25 m3), and four foundations (0,4 x 0,4 x 0,3 = 0,2 m3), each bench will need 0,5 m3.
Steel bars for reinforcing, could be around 15 Kg/m3.
In Spain, concrete price is around 50 EUR/m3, so I think in USA could be similar.
Steel price for reinforcing bars is around 0,8 EUR/Kg
So, 16 benchs wouldn´t cost more than 800 or 900 EUR (900 or 1000 $)
Another solution could be to use prefabricated factory pieces. You would need to look for a local store that have pieces in U or rectangular form that fit your needs. Pieces in U or rectangular form could be used for civil works like roads drains, by example.