whats my preamp worth?

The Blue Book gives a value of $530.

You may want to consider purchasing a membership. This will give you access to the audiogon blue book which gives high, low and average selling prices for most of the equipment you find for sale here. If you choose not to go that way, it could be an expensive experiment.

Audiogon really would prefer that you pay for a "bluebook" subscription to find that out rather than ask.

You can always check completed auction listing on a well know auction site that starts with "E".

As with all things it is worth what someone will pay now not what you paid then.