Have you tested heat inactivation of the endogenous proteases, this will be cheaper than inhibitors? Cholesterol won´t change if you cook the samples before measurement.

Yes, I have used the kit several times for cells, based on a protocol in this paper: 

http://www.jlr.org/content/47/3/605.full

Here is the relevant excerpt:

"At the end of the experiment, cell monolayers were washed with MEM-HEPES medium and lysed in 0.5 ml of a 1% sodium cholate solution in water supplemented with 10 U/ml DNase. A total of 125 μl of a reaction buffer containing 0.5% Triton X-100, 0.5 M potassium phosphate (pH 7.4), 0.25 M NaCl, and 1% sodium cholate was added to the cell lysates, and plates were shaken for 30 min at room temperature. Care must be taken to ensure complete solubilization of the cell monolayer. After heating samples at 60°C for 30 min, to inactivate enzymes that could compete with the enzymatic cholesterol assay, cholesterol was measured fluorimetrically using the Amplex Red Cholesterol Assay Kit (Molecular Probes, Eugene, OR) as described by the manufacturer. Because the experiments were conducted in the presence of an ACAT inhibitor, the cholesterol present in the cells at T0 was unesterified. However, the enzymatic kit assay for cholesterol included a cholesterol esterase and allowed the measurement of all cellular cholesterol. This permitted the quantitation of any CE incorporated from lipoproteins. The amount of cholesterol in each well was measured by comparison with a cholesterol standard curve included in each experiment. An aliquot of the cell lysates was also taken to measure cell protein by a modified Lowry method ( 15)."

http://www.jlr.org/content/47/3/605.full