Simples.com was $5,000
Simples.net was $250
Simples.pro was $50
So there you have it. Simples.biz isn't really that special to the re-seller, for the .biz extension is almost never used nowadays.
But if you're selling to an end user, I would say about $1,000 is what you may be able to get. But I personally wouldn't pay over $5 for it.
Also, I will admit, I am a noob domain appraisal. I basically just regurgitate what I hear and see other's say about similar name values. So any critiquing is much appreciated. Also, the only reason I feel the need to give my opinion in the first place, is to give some perspective of what noobs may feel the value should be.