I agree with Chuck Z that the selling price in 1975 has no bearing on current value of your trailer. If you recently acquired the trailer, then the bill of sale is the current market value. A gutted, unusable trailer isn't worth much. A fully renovated trailer ready to go can be worth $40k. Study asking prices in the Classifieds for some "comparables".
Here is the 1975 Price List from the Airstream archives. But again, it means nothing relative to current value of your particular Airstream.