Depending on condition and if it is black or silver, $350-$850

It's a black faced unit that is supposed to be in excellent condition.

On which aplication you find these la-4's best for ?

There's a thread here somewhere about a mod for them. Looks interesting.
cheers

A pair of Blackfaces in vgc with adjacent or close serial numbers (for stereo tracking) can go for between $1600 and $2000. Silver faces go for less.

On which application you find these la-4's best for ?

I like the LA4s on guitars... to me, they sound very much like a VCA type instead of an optical comp. They are good on a lot of things, acoustic and electric guitars, vocals, bass, drums. etc. Anything where you need good sounding compression..

I would try to find a pair of them, so they can be used in a stereo configuration. For any of the UREI stuff to track stereo well, it is most desirable to find units with adjacent serial numbers or at least within 10 numbers apart.. sometimes units produced on different days don't tie together well and won't track stereo accurately.