Laptops depreciate pretty quickly, but check out recent completed bids on ebay. I think that would be a fare assessment of current value.

As Z-man said, they usually won't make you pay anything if you're only $100 or so over your limit. It's just not worth their time. In fact, I have Nexus and if I'm over limit, I'll always use the line over the kiosk since with the latter I have to go see the cashier even if I'm $1 over the limit, but with an agent it's at his discretion.

As others have said, honesty is the best policy. Once you're flagged, it's really hard to get off the list. I'd sooner lie to my wife than customs!