As mentioned, ship with USPS to avoid brokerage fees. They'll only charge a $5 flat fee for handling at the border.
To determine duty check
here.
Point 92 at the link above covers guitars. Looks like duty is 6% of purchase price. So including the handling fee, you should get charged $77 at the border.
If they charge you more (and they do screw up fairly often), you can appeal and get your money back.