Well that's a bit of a bargain, ain't it?
The original fees were rumored to be in the £8m range for the 29-year-old, which actually would've been higher than what we paid to Newcastle a year and a half ago (ignoring wages, etc.), but according to the public financial records, Besiktas only paid €6m and even that will come in four installments over the next two years after an up front payment of €1.5m.
Regarding the transfer agreement with the Chelsea club over Demba Ba:
-1.5 million euros in cash
-1.5 million euros on 31.12.2014
-1.0 million euros on 30.06.2015
-1.0 million euros on 31.12.2015
-1.0 million euros on 30.06.2016
Total 6,000,000 euros transfer fee is payable.
-source: KAP via Google Translate
It's understandable that the Turkish club wanted to keep the transfer fee as low as possible, considering that they're paying Demba a base salary of €2.5m/year, plus match and performance bonuses. Fortunately for Demba, the bonuses probably don't stipulate that he has to stay on his feet for the goals to count.
All jokes aside, good luck in Turkey, Mr. Ba. You've graced us with tremendous attitude, humility, 14 goals, comment sections full of baying sheep, and several amazing memories. Here's a short, lovely video from @feroze17 to say goodbye (fair warning: Coldplay).