The placement cost is, obviously, highly dependent on volume. Assuming you are doing at least 50,000 pieces, then the placement cost might be in the region of 0.015 to 0.035 per part. But I must be honest, I'm not a manufacturing specialist and these figures are a guess based on seeing other people's numbers. The highest numbers I have seen per part in decent volume is about 5.5 cents per part, and I think the lowest figure can go below a penny, but that might be above 500K pieces. So, if these figures are roughly correct, the extra four parts might cost you a dime plus or minus.
There will also be a cost related to purchasing labor, usually a small percentage surcharge on the cost of the part which should be factored in. I've met many operations managers who hold overall BOM reduction up as a religion. They argue that fewer unique parts in an assembly has a multiplicative affect on product cost thanks to the reduction of hidden administrative costs on stocking and tracking and purchasing parts. I don't get it myself, but thought I should pass that on.
There is another issue that should be considered. The predicted yield in production is partly based on the number of solder joints and partly on how reliably the design will work with component variations (among other things). With 3 transistors and 3 resistors you have 15 solder joints, while with an IC (assuming, say, 8 pins) we have 10 solder joints. These aren't that far apart, so next I wonder which circuit will work the more reliably and tolerate the variation in components and suppliers. Often, use of an IC implies that the circuit is more complex, that is with greater design sophistication, and has been more thoroughly engineered. The individual transistors in the IC will be better matched, and it might have better temperature compensation. But I'm guessing now. I wonder how good your simpler discrete circuit will perform. Have you done any Monte Carlo on it? If there is any doubt that the circuits aren't equivalent in function and performance, then consider carefully that the downside cost of this difference could overwhelm the other costs.