The average Mucinex cash price is about $20.65 for a supply of: Tablet Extended Release 12 Hour 600 MG 30 pcs. Get our free Mucinex coupon and be eligible to receive a discount. You can reduce your Mucinex cost by up to 75% with our free Mucinex discount card. Bring it to a participating pharmacy near you to save on your prescription medication.Mucinex (guaifenesin) is used to relieve the symptoms of cough and mucus in the chest due to colds, flu, or hay fever. Mucinex may also be used for other conditions as determined by your doctor. Mucinex is an expectorant. It works by thinning mucus (phlegm) in the lungs and making it less sticky and easier to cough up. This reduces chest congestion by making coughs more productive.