These replacement toothbrush heads are $9.99 on Amazon. If you want a deal on this 16-pack, use Subscribe & Save for 5% or 10% off. Then try code 4DLNMR5L for 50% off, and clip the coupon for an additional 10% off. This brings your final price as low as $2.99.
Note that these brush heads are for select Oral-B toothbrushes.
The next time you're getting something this cheap, make sure to earn free Amazon promotional credits so you can get it almost free. These Amazon credits are easy to earn, and you can use them on just about everything.