The Czech Republic international revealed, in an interview with ctsport.cz, that he was shocked by the amount his parent club wanted for him.
He added: "Fulham asked about the availability of me and Piazon. Chelsea said two sums that Fulham said was too much for them. And it was done.
"I did not expect that amount when I had the final year of my contract. Chelsea, on the other hand, is not in the position to need to sell players at a price and get some money. It is quite understandable."
The 24-year-old is back at Craven Cottage for a second spell after helping Fulham make the Championship play-off places last season.
He has revealed the names of two other clubs that were interested in signing him, but that Fulham were always his first choice.
"I was waiting for offers and Fulham was my priority. Hull, Huddersfield, several other clubs in the Championship, but they did not. I know the Premier League is different and it would be a step forward but I have just chosen certainty."
In the same interview, Kalas admits Stamford Bridge boss Antonio Conte said "five or six words" to him in pre-season before he rejoined Fulham.
He said: "Well I saw him, that's all I can say. For the first time and for the last time, I had a chat with him in Singapore. Whatever it was, I cannot say it...five words, six words."