Mid seventies trailers aren't worth as much as early sixties trailers. Short trailers seem to be worth more than the longer ones. There was no Bambi in 76, but there was a Globetrotter 21' single axle.
Price is dependent on condition. An original trailer in good condition is always a pleasant find. A 42 year old trailer will have some needs before it is really solid and reliable.
I suggest you start searching for comparables as advertised. I would guess a nice original 76 Globetrotter might not surprise at $15k asking price if it is ready to tow (tires, brakes, lights) and ready to camp (electrical, plumbing, appliances, soft goods) are in good condition.
But do your research first. My guess is worth nothing...
WBCCI #8607 VAC Region 11
KnowledgeBase trailer renovation threads: 69 Globetrotter, 76 Sovereign, 75 Overlander, 66 Trade Wind Such fun !