They have been pursuing the 25-year-old for much of the summer and have had two bids for the Croatian turned down.
Chelsea owner Roman Abramovich is now considering whether to increase the offer to £35m but Villas-Boas realises the price must be right.
"It depends on a lot of things," said the Chelsea boss. "It depends on the club’s decision, if they are willing to go forward or not, up to which value and to decide whether they want to keep the interest in the player.
"It depends on what you have available. On the price, and availability, on the age of the player that you get, on everything. Luka, on this particular occasion, is a young player, but there would be for sure a sensible price to pay."