Help determining the value of my father's 1978 Triumph Bonneville T140E 750cc

I'm helping my father sell his 1978 Triumph Bonneville T140E 750cc, but I'm not sure what it's worth, and neither is he. I'd like to settle on a price before I start reaching out to websites and forums to list the bike for sale.

Here are a few notes about the bike that I've compiled from conversations with my father:

• All electrical systems, including lights, signals, and gauges, are fully functional.
• The fender and exhaust pipes were re-chromed years ago.
• Throttle and brake cables are relatively new and have been adjusted.
• The bike has an electronic ignition system.
• The original metric steel bolts have been replaced with American-threaded stainless steel throughout the bike
• The transmission has standard "one down, four up" gearing
• It has an in-frame oil system, and a custom bracket has been added for the oil filter.
• It has a dual-carburetor setup
• The fuel system includes a petcock to engage the main tank and a separate reserve side for additional gasoline.

The bike includes a rare, non-factory sissy bar (pictured), which was manufactured by dealerships in the 1970s. My father claims this is valuable. The seat shows some minor wear such as a small split along the seam of the leather, but it should be easy to repair. The bike has been ridden sparingly, since my father was already an old man when he repaired it and couldn't ride it often. It's sat in his garage for nearly the entire time, unused.

If anyone knows what this might be worth, it would be much appreciated. I'm just looking for a ballpark, I want to make sure my father gets a reasonable price while still giving a deal to whoever buys it.

https://imgur.com/a/RenDIcJ