Lowering kits can vary in price quite a lot, ranging from around $100 to $700. The labor will cost you another $200-$800, depending on the mechanic’s rates and expertise.
You should be aware then improper lowering can cause serious issues for your car, so you should avoid simply going with the cheapest option. In general, for decent quality parts and decent labor, you should expect to pay around $1,000 in total.