He told the Daily Record: "They say Rangers have not had much money until now.
"But I find that hard to understand because they get 50,000 people every other week and it is a very big club.
"We are being obliged to contemplate transfers of £6m, £8m and that type of figure, so it seems strange a club of that stature can only afford what they have offered us.
"I am perfectly happy to keep Steven. He is a good player and I would be very disappointed if the club allowed him to go for a sum of money that is below his worth."