Depends how badly you want to get the achievement. They are very rare so if that's what they're going for these days (I'm on xbox EU and that sounds about the ballpark figure) then you're unlikely to get one much cheaper.

You could spend ages checking every trader every day for weeks on end and maybe you could save yourself 10, 20, 50k or maybe not so it depends what value you put on your time.

On the PC/NA, they sell for around 50K give or take. I have seen them for less.

But it is one of the rarest drops in the game. I am a motif hunting fool and have found and sold 100's of motifs (I parked 5 Mule Alts in Wayrest and farmed 1 to 2 a day on each one for about a year so a 1000 would not surprise me if it was that many). I have checked almost every container I came across in the past 2.5 years and since crafting bags, Auto Loot. Only Imperial Motif I found was during release week when they dropped like Blue Motifs. Of course I found it on my Imperial, and figured I did not need it then as it was so easy to find.

Not one since. Both of mine have been bought in the Guild Stores.

Probably varies given that it is a pretty rare drop. I think I've gotten 2 in my entire time playing the game and I've played pretty regularly since launch.