For common chips, look them up on eBay's search for sold items.

This is more accurate and current than the guide's pricing. The downside is that it takes a lot more work than flipping through a book. A benefit is that you have pictures of each sold chip to evaluate table worn vs cage fresh, in the price they sold for.

You do have to pay attention to the shipping cost, free vs., say, $3. So you see a chip sell for $6 w/ free shipping, and the same beast sell for $3 w/ $3 shipping, and a few others of the same chip in the same range... Chip is "worth" $5 or $6.

Also, look at the current listings. You see 5 copies in the same condition as your chip, that you determined with the above method is worth $6, listed in the $4 neighborhood. Want it to sell first of the available copies of the chip? Your $6 value chip is now worth $3.75.