I am biased of course since I own one, but I think I can still give some insight to the ones who are confused why so many people seem to be willing to pay top dollar for this weapon.
The reason this weapon is valued highly is because it performs incredibly for its skill level. The eco is very good as well, but even if we put that aside completely there is just no better UL gun other than the terminator in its skill range. Since there are a lot of players out there between level 35-50, the (L) guns in this skill range carry a hefty markup. Even the players that ignore eco altogether usually believe that excessive markup is a money pit that should be avoided if possible, and I agree.
Let's do some numbers then shall we
The following is a list of similar dps weapons that aren't too far away from the skill req of the Spirit mk.II TEN, and the amount of peds that you spend in markup for an hour of shooting. As you can see, if you want range you will have to pay for it dearly in markup.
Isis Zero-Four 20ped/hour
Svempa x1 20ped/hour
Isis Zero-Five 30ped/hour
Spirit mk.III 40ped/hour
AL26 65ped/hour
If you hunt for a few hours per day and/or plan on reaching a high level in hunting you will eventually spend more money in markup than the cost to buy a decent UL SIB. Skilling from level 35-75 where you can start getting some cheaper (L) guns requires quite a bit of peds cycled, and I don't think the high level (L) guns will stay low markup forever.
To RickEngland, you want 100+ dmg unamped? Well you're looking at a lot higher level weapons than this and I understand your disinterest