The value of light bars is all over the map - the last one I bought was only $50, and the one before that was $65 - but these are both rotator bars with halogen bulbs. LED bars are worth more - if they're Whelen, and you need red/blue LEDs, they seem to go for about $250 on up. Amber or green are a lot harder to scrounge, and they seem to start at about $350. Scratched or hazy domes can be polished with automotive rubbing compound.
If those prices seem high, consider that a new LED bar runs about $2000 (+/-) for American made. Chinese stuff is a lot cheaper, but the quality is all over the place.
You might do OK donating them to the FD in exchange for a future favor.