hydro- boost brake using g-m astro unit

No clue on what they are worth.
But here is a new refurb I would guess at least half that price. (Core charge of $62)
https://www.summitracing.com/parts/...kOFyJCJ9rWiqwVJqM93WORUjBpDzTLQxoC9JkQAvD_BwE

As far as I know they are all the same internally, some have a bigger canisters. The canister holds spare pressure just in case of emerge situations.
Mounting is likely different between every make.