The following are estimates for a single-unit tree box filter:

Cost
(Including two years of maintenance, filter material, and plants)
$6,000 per unit per 1/4 acre of impervious surface

Installation
Varies with each site; generally the same as a precast inlet structure
$1500 per unit

Annual maintenance
Performed by the Manufacturer:  $500 per unit  (less for multiple units)
Performed by the Owner: $100 per unit