From what I've found, DeAngelo Hall's cap figures are nowhere near the 6 for $25M that you claim. Maybe that is the maximum that he could make. This is what I found.
Signed by Falcons to a six-year contract. Terms of his deal were undisclosed, but he has base salaries of: $230,000 (2004); $305,000 (2005); $385,000 (2006); $460,000 (2007); $570,000 (2008); and $957,500 (2009). He also received a $12 million signing bonus, giving him cap figures of: $2.23 million (2004); $2.305 million (2005); $2.385 million (2006); $2.46 million (2007); $2.57 million (2008); and $2.9575 million (2009).
That is much more manageable that what Clements is likely to want in free agency next year. Ken Lucas, who is not on the same level as Clements, got a 6 year $36M deal with $13M in signing bonus money in free agency. Gary Baxter and Anthony Henry also got double digit signing bonus's to go along with 6 year ~$30M deals.
So, what will Clements demand on the open market? I dont know for sure, but it wouldn't suprise me if he wanted $15-18M in bonus money and 6 years for $42M at the minimum. So the trade for the #9 pick would definitely be the cheaper alternative.
Saying all of this, I'm not sure that Clements is not worth all this money. He's 26 years old, and on the cusp of being the best CB in the league, so if we can somehow keep him on board beyond this year, get it done. But, I definitely do have a problem if we are just keeping him for this one year and then letting him leave in free agency for nothing. The Bills must either lock up Clements or trade him now.