Why look at BT rating? That shows you nothing, trust me. There are many players whose ratings are hugely inflated. I have a 19 yr old strong bowler with mediocre secondaries and his BTR is only 8000, while other respectables with competant secondaries have a BTR of 10,000 or so.
Also, your estimates are often wrong. I bought a proficient teenage bowler with feeble secondaries for ?200,000, and sold Howard Lakeman, the one whom you said wasn't worth ?100,000, for ?130,000.