As above... $500 is about what it's worth. That ski, redy to run is worth $2k... maybe $2500 in the spring time, if it's really clean. If you have to take it to a mechanic to have it fixed... you are looking at $1500. So... there's where your $500 is coming from. But... Since it doesn't run... there's no real way for a potential buyer to know if there are other issues. (burnt up electrical, etc)
Also... as above... you can make good money parting it out, if it's clean. The trailer is $300 to $500, the MPEM is $100~$120, There is $300 in random electrical parts... seat, hood, pump... even the good engien parts will sell. (case, head, carbs)
If you do it right... there's $2k in parts. (Assuming super clean)
But... if it is as clean as you say... and you like the ski... then put an engine in it, and DO IT YOURSELF !!!!!!! This is why a lot of dealers don't take in older skis. They know that the labor winds up exceeding the value of the PWC.
As far as "My ski boat is a 92, my fishing boat is a 88.".....
Well... they didn't spend 90% of their life turning 7000 RPM, and jumping out of the water, bouncing off the rev-limiter. Not to mention... they aren't even close to the same power levels. (hp/cc) PWC engines are exceeding the hp/cc limits of even the best "Super cars" out there. Your 717 engine is at 119 hp/L !!!!!! I'm sure your other boats are around the 60~80 HP/L range. Just an FYI... Mercury takes 1500 cc's to make a 120hp engine. Then jumps up to 2500cc's to get the 175hp. Because of that... they can run with lower quality oil, and with loose tolerances. In turn... living a longer life.
I'm not trying to be harsh... but I get the feeling you are looking for a real answer, when the engine has just simply lived it's life.
Just one last thought.... This isn't common to just Seadoo. ALL, PWC engines are like this. AND, your 720 engine is one of the most robust. (Along with the Polaris 1200/900, Yamaha 701/650, and Kawi's 550) Some of the worst are Yamaha's 1200/1300/800 and seadoo's 951. (They only get 200 to 250 hrs, if well maintained)